A combined proteomics/genomics approach links hepatitis C virus infection with nonsense-mediated mRNA decay

Cell-intrinsic regulation of HBV RNAs by the nonsense-mediated mRNA decay pathway controls viral replication

Hepatitis B virus (HBV) is a causative agent for chronic liver hepatitis, which confers risk for liver cirrhosis and hepatocellular carcinoma. Among key viral transcripts, HBV pregenome RNA (pgRNA) is indispensable for viral replication, and therefore, quality control of pgRNA is critical for the HBV life cycle. Here, we revealed degradation of HBV RNAs by the nonsense-mediated mRNA decay (NMD) pathway, a host surveillance system of RNA quality. Degradation kinetics of the HBV RNAs indicated that pgRNA, 2.4 knt RNA, and 2.1 knt RNA were targets of the NMD pathway and also interacted robustly with phosphorylated UPF1 but not X RNA. Northern blotting showed that decay of the viral NMD candidates was also delayed in NMD-deficient cells. In contrast, NMD depletion promoted the formation of capsids containing genomic DNA and exhibiting antigen production. Our data strongly suggest that the NMD pathway inspects HBV transcripts to regulate HBV replication as an intrinsic antiviral defense.

The dynamic interactions between virus infections and nonsense-mediated decay

Humans are continuously exposed to a wide array of viruses that cause a significant amount of morbidity and mortality worldwide. Over recent years, the evolutionarily conserved host RNA degradation pathway nonsense-mediated decay (NMD) has emerged as a broad antiviral defense mechanism that controls infection of a variety of RNA and DNA viruses. Besides regulating the abundance of host transcripts, NMD directly destabilizes virus genomic RNA, replication intermediates, and viral transcripts to interfere with replication. In turn, viruses have evolved strategies to modulate cellular NMD activity or repurpose NMD factors to facilitate their replication. In this review, we describe our current understanding of the role of NMD in controlling virus infections as well as the strategies employed by viruses to interfere with NMD.

PYM1 limits non-canonical Exon Junction Complex occupancy in a gene architecture dependent manner to tune mRNA expression

The Exon Junction Complex (EJC) deposited upstream of exon-exon junctions during pre-mRNA splicing in the nucleus remains stably bound to RNA to modulate mRNA fate at multiple post-transcriptional steps until its disassembly during translation. Here, we investigated two EJC disassembly mechanisms in human embryonic kidney 293 (HEK293) cells, one mediated by PYM1, a factor that can bind both the ribosome and the RBM8A/MAGOH heterodimer of the EJC core, and another by the elongating ribosome itself. We find that EJCs lacking PYM1 interaction show no defect in translation-dependent disassembly but is required for translation-independent EJC destabilization. Surprisingly, PYM1 interaction deficient EJCs are enriched on sites away from the canonical EJC binding position including on transcripts without introns or with fewer and longer exons. Acute reduction of PYM1 levels in HEK293 cells results in a modest inhibition of nonsense-mediated mRNA decay and stabilization of mRNAs that localize to endoplasmic reticulum associated TIS-granules and are characterized by fewer and longer exons. We confirmed the previously reported PYM1-flavivirus capsid protein interaction and found that human cells expressing the capsid protein or infected with flaviviruses show similar changes in gene expression as upon PYM1 depletion. Thus, PYM1 acts as an EJC specificity factor that is hijacked by flaviviruses to alter global EJC occupancy and reshape host cell mRNA regulation.

Selective Degradation of Cucumber Mosaic Virus RNA3 by Nonsense-Mediated Decay Benefits Viral Early Infection

Nonsense-mediated mRNA decay (NMD) is a critical RNA quality control system in eukaryotes, also playing a role in defending against viral infections. However, research has primarily focused on nonsegmented viruses. To investigate the interaction between NMD and segmented RNA viruses, we used cucumber mosaic virus (CMV), which possesses a tripartite, single-stranded, positive-sense RNA genome. Agroinfiltration assays were performed to assess how CMV RNA segments, or their variants, respond to NMD. We found that CMV genomic segments (RNAs 1-3) exhibit distinct responses to NMD. Specifically, RNA3, which serves as the translation template of the movement protein (MP), is selectively degraded by NMD, unlike RNA1 and RNA2, which encode viral replicase components. This degradation is triggered by the coat protein (CP) sequence and can be mitigated by the trans-expression of the 1a replicase or CP. The 1a protein requires its specific interaction with the Box-B motif of RNA3 to avoid NMD. Importantly, compromising NMD reduces CMV infection during the early stages, suggesting that NMD-mediated RNA3 degradation facilitates initial viral replication. This is supported by observations that MP expression in trans negatively regulates viral RNA replication. We propose a model to illustrate the molecular interplay between NMD and CMV, emphasising the implications of genomic segmentation in NMD-virus interactions.

HBV Remodels PP2A Complexes to Rewire Kinase Signaling in Hepatocellular Carcinoma

Hepatitis B virus (HBV) infections promote liver cancer initiation by inducing inflammation and cellular stress. Despite a primarily indirect effect on oncogenesis, HBV is associated with a recurrent genomic phenotype in hepatocellular carcinoma (HCC), suggesting that it impacts the biology of established HCC. Characterization of the interaction of HBV with host proteins and the mechanistic contributions of HBV to HCC initiation and maintenance could provide insights into HCC biology and uncover therapeutic vulnerabilities. In this study, we used affinity purification mass spectrometry to comprehensively map a network of 145 physical interactions between HBV and human proteins in HCC. A subset of the host factors targeted by HBV proteins were preferentially mutated in non-HBV-associated HCC, suggesting that their interaction with HBV influences HCC biology. HBV interacted with proteins involved in mRNA splicing, mitogenic signaling, and DNA repair, with the latter set interacting with the HBV oncoprotein X (HBx). HBx remodeled the PP2A phosphatase complex by excluding striatin regulatory subunits from the PP2A holoenzyme, and the HBx effects on PP2A caused Hippo kinase activation. In parallel, HBx activated mTOR complex 2, which can prevent YAP degradation. mTOR complex 2-mediated upregulation of YAP was observed in human HCC specimens and mouse HCC models and could be targeted with mTOR kinase inhibitors. Thus, HBV interaction with host proteins rewires HCC signaling rather than directly activating mitogenic pathways, providing an alternative paradigm for the cellular effects of a tumor-promoting virus. Significance: Integrative proteomic and genomic analysis of HBV/host interactions illuminated modifiers of hepatocellular carcinoma behavior and key signaling mechanisms in advanced disease, which suggested that HBV may have therapeutically actionable effects.

The SARS-CoV-2 nucleocapsid protein interferes with the full enzymatic activation of UPF1 and its interaction with UPF2

The nonsense-mediated mRNA decay (NMD) pathway triggers the degradation of defective mRNAs and governs the expression of mRNAs with specific characteristics. Current understanding indicates that NMD is often significantly suppressed during viral infections to protect the viral genome. In numerous viruses, this inhibition is achieved through direct or indirect interference with the RNA helicase UPF1, thereby promoting viral replication and enhancing pathogenesis. In this study, we employed biochemical, biophysical assays and cellular investigations to explore the interplay between UPF1 and the nucleocapsid (Np) protein of SARS-CoV-2. We evaluated their direct interaction and its impact on inhibiting cellular NMD. Furthermore, we characterized how this interaction affects UPF1's enzymatic function. Our findings demonstrate that Np inhibits the unwinding activity of UPF1 by physically obstructing its access to structured nucleic acid substrates. Additionally, we showed that Np binds directly to UPF2, disrupting the formation of the UPF1/UPF2 complex essential for NMD progression. Intriguingly, our research also uncovered a surprising pro-viral role of UPF1 and an antiviral function of UPF2. These results unveil a novel, multi-faceted mechanism by which SARS-CoV-2 evades the host's defenses and manipulates cellular components. This underscores the potential therapeutic strategy of targeting Np-UPF1/UPF2 interactions to treat COVID-19.

N Protein of Tomato Spotted Wilt Virus Proven to Be Antagonistic Against Tomato Yellow Leaf Curl Virus in Nicotiana benthamiana

Two phylogenetically unrelated viruses transmitted by different insect vectors, tomato spotted wilt virus (TSWV) and tomato yellow leaf curl virus (TYLCV), are major threats to tomato and other vegetable production. Although co-infections of TSWV and TYLCV on the same host plant have been reported on numerous occasions, there is still lack of research attempting to elucidate the mechanisms underlying the relationship between two viruses when they coexist in the same tomato or other plants. After assessing the effect of four TSWV-coded proteins on suppressing TYLCV in TSWV N transgenic Nicotiana benthamiana seedlings, the TSWV N protein proved to be effective in reducing TYLCV quantity and viral symptoms. Western blot analysis indicated that TSWV N was involved in down-regulating the expression level of the V1, C3, and C4 proteins of TYLCV, among which V1 was the most significantly suppressed one. Moreover, TSWV N was confirmed to reduce TYLCV V1 within both nucleus and cytoplasm, but a greater suppression was observed in cytoplasm. The co-immunoprecipitation and mass spectrometry identified 244 differential proteins from the TYLCV-infected TSWV N transgenic N. benthamiana seedling. These proteins pertaining to energy metabolism pathways were enriched, suggesting that TSWV N could inhibit TYLCV through competing for energy or regulating energy-related metabolism. The evidence presented here offers a novel perspective that will facilitate a comprehensive understanding of virus-virus and virus-host interactions, as well as a potential strategy for plant virus control through using TSWV N in the near future.

Cutting through the stress: RNA decay pathways at the endoplasmic reticulum

The endoplasmic reticulum (ER) is central to the processing of luminal, transmembrane, and secretory proteins, and maintaining a functional ER is essential for organismal physiology and health. Increased protein-folding load on the ER causes ER stress, which activates quality control mechanisms to restore ER function and protein homeostasis. Beyond protein quality control, mRNA decay pathways have emerged as potent ER fidelity regulators, but their mechanistic roles in ER quality control and their interrelationships remain incompletely understood. Herein, we review ER-associated RNA decay pathways - including regulated inositol-requiring enzyme 1α (IRE1α)-dependent mRNA decay (RIDD), nonsense-mediated mRNA decay (NMD), and Argonaute-dependent RNA silencing - in ER homeostasis, and highlight the intricate coordination of ER-targeted RNA and protein decay mechanisms and their association with antiviral defense.

Paramyxovirus matrix proteins modulate host cell translation via exon-junction complex interactions in the cytoplasm

Viruses have evolved myriad strategies to exploit the translation machinery of host cells to potentiate their replication. However, how paramyxovirus (PMVs) modulate cellular translation for their own benefit has not been systematically examined. Utilizing puromycylation labeling, overexpression of individual viral genes, and infection with wild-type virus versus its gene-deleted counterpart, we found that PMVs significantly inhibit host cells' nascent peptide synthesis during infection, with the viral matrix being the primary contributor to this effect. Using the rNiV-NPL replicon system, we discovered that the viral matrix enhances viral protein translation without affecting viral mRNA transcription and suppresses host protein expression at the translational level. Polysome profile analysis revealed that the HPIV3 matrix promotes the association of viral mRNAs with ribosomes, thereby enhancing their translation efficiency during infection. Intriguingly, our NiV-Matrix interactome identified the core exon-junction complex (cEJC), critical for mRNA biogenesis, as a significant component that interacts with the paramyxoviral matrix predominantly in the cytoplasm. siRNA knockdown of eIF4AIII simulated the restriction of cellular functions by the viral matrix, leading to enhanced viral gene translation and a reduction in host protein synthesis. Moreover, siRNA depletion of cEJC resulted in a 2-3 log enhancement in infectious virus titer for various PMVs but not SARS-CoV-2, enterovirus D68, or influenza virus. Our findings characterize a host translational interference mechanism mediated by viral matrix and host cEJC interactions. We propose that the PMV matrix redirects ribosomes to translate viral mRNAs at the expense of host cell transcripts, enhancing viral replication, and thereby enhancing viral replication. These insights provide a deeper understanding of the molecular interactions between paramyxoviruses and host cells, highlighting potential targets for antiviral strategies.

I'm Walking into Spiderwebs: Making Sense of Protein-Protein Interaction Data

Protein-protein interactions (PPIs) are at the heart of the molecular landscape permeating life. Proteomics studies can explore this protein interaction landscape using mass spectrometry (MS). Thanks to their high sensitivity, mass spectrometers can easily identify thousands of proteins within a single sample, but that same sensitivity generates tangled spiderwebs of data that hide biologically relevant findings. So, what does a researcher do when she finds herself walking into spiderwebs? In a field focused on discovery, MS data require rigor in their analysis, experimental validation, or a combination of both. In this Review, we provide a brief primer on MS-based experimental methods to identify PPIs. We discuss approaches to analyze the resulting data and remove the proteomic background. We consider the advantages between comprehensive and targeted studies. We also discuss how scoring might be improved through AI-based protein structure information. Women have been essential to the development of proteomics, so we will specifically highlight work by women that has made this field thrive in recent years.

Mapping protein-protein interactions by mass spectrometry

Protein-protein interactions (PPIs) are essential for numerous biological activities, including signal transduction, transcription control, and metabolism. They play a pivotal role in the organization and function of the proteome, and their perturbation is associated with various diseases, such as cancer, neurodegeneration, and infectious diseases. Recent advances in mass spectrometry (MS)-based protein interactomics have significantly expanded our understanding of the PPIs in cells, with techniques that continue to improve in terms of sensitivity, and specificity providing new opportunities for the study of PPIs in diverse biological systems. These techniques differ depending on the type of interaction being studied, with each approach having its set of advantages, disadvantages, and applicability. This review highlights recent advances in enrichment methodologies for interactomes before MS analysis and compares their unique features and specifications. It emphasizes prospects for further improvement and their potential applications in advancing our knowledge of PPIs in various biological contexts.

Proximity labeling of host factor ANXA3 in HCV infection reveals a novel LARP1 function in viral entry

Hepatitis C virus (HCV) infection is tightly connected to the lipid metabolism with lipid droplets (LDs) serving as assembly sites for progeny virions. A previous LD proteome analysis identified annexin A3 (ANXA3) as an important HCV host factor that is enriched at LDs in infected cells and required for HCV morphogenesis. To further characterize ANXA3 function in HCV, we performed proximity labeling using ANXA3-BioID2 as bait in HCV-infected cells. Two of the top proteins identified proximal to ANXA3 during HCV infection were the La-related protein 1 (LARP1) and the ADP ribosylation factor-like protein 8B (ARL8B), both of which have been previously described to act in HCV particle production. In follow-up experiments, ARL8B functioned as a pro-viral HCV host factor without localizing to LDs and thus likely independent of ANXA3. In contrast, LARP1 interacts with HCV core protein in an RNA-dependent manner and is translocated to LDs by core protein. Knockdown of LARP1 decreased HCV spreading without altering HCV RNA replication or viral titers. Unexpectedly, entry of HCV particles and E1/E2-pseudotyped lentiviral particles was reduced by LARP1 depletion, whereas particle production was not altered. Using a recombinant vesicular stomatitis virus (VSV)ΔG entry assay, we showed that LARP1 depletion also decreased entry of VSV with VSV, MERS, and CHIKV glycoproteins. Therefore, our data expand the role of LARP1 as an HCV host factor that is most prominently involved in the early steps of infection, likely contributing to endocytosis of viral particles through the pleiotropic effect LARP1 has on the cellular translatome.

A foundational atlas of autism protein interactions reveals molecular convergence

Translating high-confidence (hc) autism spectrum disorder (ASD) genes into viable treatment targets remains elusive. We constructed a foundational protein-protein interaction (PPI) network in HEK293T cells involving 100 hcASD risk genes, revealing over 1,800 PPIs (87% novel). Interactors, expressed in the human brain and enriched for ASD but not schizophrenia genetic risk, converged on protein complexes involved in neurogenesis, tubulin biology, transcriptional regulation, and chromatin modification. A PPI map of 54 patient-derived missense variants identified differential physical interactions, and we leveraged AlphaFold-Multimer predictions to prioritize direct PPIs and specific variants for interrogation in Xenopus tropicalis and human forebrain organoids. A mutation in the transcription factor FOXP1 led to reconfiguration of DNA binding sites and altered development of deep cortical layer neurons in forebrain organoids. This work offers new insights into molecular mechanisms underlying ASD and describes a powerful platform to develop and test therapeutic strategies for many genetically-defined conditions.

Hepatitis C virus infects and perturbs liver stem cells

IMPORTANCE

The hepatitis C virus (HCV) causes liver disease, affecting millions. Even though we have effective antivirals that cure HCV, they cannot stop terminal liver disease. We used an adult stem cell-derived liver organoid system to understand how HCV infection leads to the progression of terminal liver disease. Here, we show that HCV maintains low-grade infections in liver organoids for the first time. HCV infection in liver organoids leads to transcriptional reprogramming causing cancer cell development and altered immune response. Our finding shows how HCV infection in liver organoids mimics HCV infection and patient pathogenesis. These results reveal that HCV infection in liver organoids contributes to liver disease progression.

Mitochondrial Oxidative Phosphorylation in Viral Infections

Mitochondria have been identified as the "powerhouse" of the cell, generating the cellular energy, ATP, for almost seven decades. Research over time has uncovered a multifaceted role of the mitochondrion in processes such as cellular stress signaling, generating precursor molecules, immune response, and apoptosis to name a few. Dysfunctional mitochondria resulting from a departure in homeostasis results in cellular degeneration. Viruses hijack host cell machinery to facilitate their own replication in the absence of a bonafide replication machinery. Replication being an energy intensive process necessitates regulation of the host cell oxidative phosphorylation occurring at the electron transport chain in the mitochondria to generate energy. Mitochondria, therefore, can be an attractive therapeutic target by limiting energy for viral replication. In this review we focus on the physiology of oxidative phosphorylation and on the limited studies highlighting the regulatory effects viruses induce on the electron transport chain.

SARS-CoV-2 helicase might interfere with cellular nonsense-mediated RNA decay: insights from a bioinformatics study

BACKGROUND

Viruses employ diverse strategies to interfere with host defense mechanisms, including the production of proteins that mimic or resemble host proteins. This study aimed to analyze the similarities between SARS-CoV-2 and human proteins, investigate their impact on virus-host interactions, and elucidate underlying mechanisms.

RESULTS

Comparing the proteins of SARS-CoV-2 with human and mammalian proteins revealed sequence and structural similarities between viral helicase with human UPF1. The latter is a protein that is involved in nonsense-mediated RNA decay (NMD), an mRNA surveillance pathway which also acts as a cellular defense mechanism against viruses. Protein sequence similarities were also observed between viral nsp3 and human Poly ADP-ribose polymerase (PARP) family of proteins. Gene set enrichment analysis on transcriptomic data derived from SARS-CoV-2 positive samples illustrated the enrichment of genes belonging to the NMD pathway compared with control samples. Moreover, comparing transcriptomic data from SARS-CoV-2-infected samples with transcriptomic data derived from UPF1 knockdown cells demonstrated a significant overlap between datasets.

CONCLUSIONS

These findings suggest that helicase/UPF1 sequence and structural similarity might have the ability to interfere with the NMD pathway with pathogenic and immunological implications.

Introduction of Dicistrovirus IRESs into UAS/SV40-polyA constructs results in premature polyadenylation and strong overexpression of the upstream ORF in Drosophila animals

To evaluate the properties of insect virus internal ribosomal entry sites (IRESs) for protein expression in Drosophila, we have introduced Cricket Paralysis virus (CrPV) and Drosophila C virus (DCV) IRESs into UAS/SV40-polyA vector. We found that introduction of IRESs induce premature polyadenylation, resulting in both truncation of the mRNA, and an increase in mRNA levels of approximately 40-fold. The increase in mRNA levels was accompanied by increased resistance to nonsense-mediated mRNA decay (NMD)-mediated degradation. Our results suggest that premature polyadenylation increases mRNA stability in the SV40 polyadenylation site-containing constructs, suggesting a novel method for robust overexpression of transgenes in Drosophila.

Proteomic and genetic analyses of influenza A viruses identify pan-viral host targets

Influenza A Virus (IAV) is a recurring respiratory virus with limited availability of antiviral therapies. Understanding host proteins essential for IAV infection can identify targets for alternative host-directed therapies (HDTs). Using affinity purification-mass spectrometry and global phosphoproteomic and protein abundance analyses using three IAV strains (pH1N1, H3N2, H5N1) in three human cell types (A549, NHBE, THP-1), we map 332 IAV-human protein-protein interactions and identify 13 IAV-modulated kinases. Whole exome sequencing of patients who experienced severe influenza reveals several genes, including scaffold protein AHNAK, with predicted loss-of-function variants that are also identified in our proteomic analyses. Of our identified host factors, 54 significantly alter IAV infection upon siRNA knockdown, and two factors, AHNAK and coatomer subunit COPB1, are also essential for productive infection by SARS-CoV-2. Finally, 16 compounds targeting our identified host factors suppress IAV replication, with two targeting CDK2 and FLT3 showing pan-antiviral activity across influenza and coronavirus families. This study provides a comprehensive network model of IAV infection in human cells, identifying functional host targets for pan-viral HDT.

Hepatitis C virus alters the morphology and function of peroxisomes

Despite the introduction of effective treatments for hepatitis C in clinics, issues remain regarding the liver disease induced by chronic hepatitis C virus (HCV) infection. HCV is known to disturb the metabolism of infected cells, especially lipid metabolism and redox balance, but the mechanisms leading to HCV-induced pathogenesis are still poorly understood. In an APEX2-based proximity biotinylation screen, we identified ACBD5, a peroxisome membrane protein, as located in the vicinity of HCV replication complexes. Confocal microscopy confirmed the relocation of peroxisomes near HCV replication complexes and indicated that their morphology and number are altered in approximately 30% of infected Huh-7 cells. Peroxisomes are small versatile organelles involved among other functions in lipid metabolism and ROS regulation. To determine their importance in the HCV life cycle, we generated Huh-7 cells devoid of peroxisomes by inactivating the PEX5 and PEX3 genes using CRISPR/Cas9 and found that the absence of peroxisomes had no impact on replication kinetics or infectious titers of HCV strains JFH1 and DBN3a. The impact of HCV on peroxisomal functions was assessed using sub-genomic replicons. An increase of ROS was measured in peroxisomes of replicon-containing cells, correlated with a significant decrease of catalase activity with the DBN3a strain. In contrast, HCV replication had little to no impact on cytoplasmic and mitochondrial ROS, suggesting that the redox balance of peroxisomes is specifically impaired in cells replicating HCV. Our study provides evidence that peroxisome function and morphology are altered in HCV-infected cells.

New Opportunities to Individualize Frontline Therapy in Advanced Stages of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a leading cause of cancer death globally and is rising in incidence. Until recently, treatment options for patients with advanced stages of HCC have been limited to antiangiogenic therapies with modest improvements in overall survival. The emerging role of immunotherapy with immune checkpoint inhibitors (ICI) in oncology has led to a rapid expansion in treatment options and improvements in outcomes for patients with advanced stages of HCC. Recent clinical trials have shown meaningful survival improvement in patients treated with the combination of bevacizumab and atezolizumab, as well as with the combination of tremelimumab with durvalumab, resulting in regulatory approvals of these regimens as frontline therapy. Beyond improvements in overall survival, ICI-based combination regimens achieve higher rates of durable treatment response than multikinase inhibitors and have favorable side effect profiles. With the emergence of doublet anti-angiogenic and immune checkpoint inhibitor (ICI) and dual ICI combinations, individualized therapy is now possible for patients based on co-morbidity profiles and other factors. These more potent systemic therapies are also being tested in earlier stages of disease and in combination with loco-regional therapies such as trans-arterial chemoembolization and stereotactic body radiotherapy. We summarize these advances and emerging therapeutic combinations currently in clinical trials.

Molecular Interaction of Nonsense-Mediated mRNA Decay with Viruses

The virus–host interaction is dynamic and evolutionary. Viruses have to fight with hosts to establish successful infection. Eukaryotic hosts are equipped with multiple defenses against incoming viruses. One of the host antiviral defenses is the nonsense-mediated mRNA decay (NMD), an evolutionarily conserved mechanism for RNA quality control in eukaryotic cells. NMD ensures the accuracy of mRNA translation by removing the abnormal mRNAs harboring pre-matured stop codons. Many RNA viruses have a genome that contains internal stop codon(s) (iTC). Akin to the premature termination codon in aberrant RNA transcripts, the presence of iTC would activate NMD to degrade iTC-containing viral genomes. A couple of viruses have been reported to be sensitive to the NMD-mediated antiviral defense, while some viruses have evolved with specific cis-acting RNA features or trans-acting viral proteins to overcome or escape from NMD. Recently, increasing light has been shed on the NMD–virus interaction. This review summarizes the current scenario of NMD-mediated viral RNA degradation and classifies various molecular means by which viruses compromise the NMD-mediated antiviral defense for better infection in their hosts.

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.

Tau-induced deficits in nonsense-mediated mRNA decay contribute to neurodegeneration

INTRODUCTION

While brains of patients with Alzheimer's disease and related tauopathies have evidence of altered RNA processing, we lack a mechanistic understanding of how altered RNA processing arises in these disorders and if such changes are causally linked to neurodegeneration.

METHODS

Using Drosophila melanogaster models of tauopathy, we find that overall activity of nonsense-mediated mRNA decay (NMD), a key RNA quality-control mechanism, is reduced. Genetic manipulation of NMD machinery significantly modifies tau-induced neurotoxicity, suggesting that deficits in NMD are causally linked to neurodegeneration. Mechanistically, we find that deficits in NMD are a consequence of aberrant RNA export and RNA accumulation within nuclear envelope invaginations in tauopathy. We identify a pharmacological activator of NMD that suppresses neurodegeneration in tau transgenic Drosophila, indicating that tau-induced deficits in RNA quality control are druggable.

DISCUSSION

Our studies suggest that NMD activators should be explored for their potential therapeutic value to patients with tauopathies.

Understanding Insulin in the Age of Precision Medicine and Big Data: Under-Explored Nature of Genomics

Insulin is amongst the human genome's most well-studied genes/proteins due to its connection to metabolic health. Within this article, we review literature and data to build a knowledge base of Insulin (INS) genetics that influence transcription, transcript processing, translation, hormone maturation, secretion, receptor binding, and metabolism while highlighting the future needs of insulin research. The INS gene region has 2076 unique variants from population genetics. Several variants are found near the transcriptional start site, enhancers, and following the INS transcripts that might influence the readthrough fusion transcript INS-IGF2. This INS-IGF2 transcript splice site was confirmed within hundreds of pancreatic RNAseq samples, lacks drift based on human genome sequencing, and has possible elevated expression due to viral regulation within the liver. Moreover, a rare, poorly characterized African population-enriched variant of INS-IGF2 results in a loss of the stop codon. INS transcript UTR variants rs689 and rs3842753, associated with type 1 diabetes, are found in many pancreatic RNAseq datasets with an elevation of the 3'UTR alternatively spliced INS transcript. Finally, by combining literature, evolutionary profiling, and structural biology, we map rare missense variants that influence preproinsulin translation, proinsulin processing, dimer/hexamer secretory storage, receptor activation, and C-peptide detection for quasi-insulin blood measurements.

WTAP Targets the METTL3 m6A-Methyltransferase Complex to Cytoplasmic Hepatitis C Virus RNA to Regulate Infection

Modification of the hepatitis C virus (HCV) positive-strand RNA genome by N6-methyladenosine (m⁶A) regulates the viral life cycle. This life cycle takes place solely in the cytoplasm, while m⁶A addition on cellular mRNA takes place in the nucleus. Thus, the mechanisms by which m⁶A is deposited on the viral RNA have been unclear. In this work, we find that m⁶A modification of HCV RNA by the m⁶A-methyltransferase proteins methyltransferase-like 3 and 14 (METTL3 and METTL14) is regulated by Wilms' tumor 1-associating protein (WTAP). WTAP, a predominantly nuclear protein, is an essential member of the cellular mRNA m⁶A-methyltransferase complex and known to target METTL3 to mRNA. We found that HCV infection induces localization of WTAP to the cytoplasm. Importantly, we found that WTAP is required for both METTL3 interaction with HCV RNA and m⁶A modification across the viral RNA genome. Further, we found that WTAP, like METTL3 and METTL14, negatively regulates the production of infectious HCV virions, a process that we have previously shown is regulated by m⁶A. Excitingly, WTAP regulation of both HCV RNA m⁶A modification and virion production was independent of its ability to localize to the nucleus. Together, these results reveal that WTAP is critical for HCV RNA m⁶A modification by METTL3 and METTL14 in the cytoplasm. IMPORTANCE Positive-strand RNA viruses such as HCV represent a significant global health burden. Previous work has described that HCV RNA contains the RNA modification m⁶A and how this modification regulates viral infection. Yet, how this modification is targeted to HCV RNA has remained unclear due to the incompatibility of the nuclear cellular processes that drive m⁶A modification with the cytoplasmic HCV life cycle. In this study, we present evidence for how m⁶A modification is targeted to HCV RNA in the cytoplasm by a mechanism in which WTAP recruits the m⁶A-methyltransferase METTL3 to HCV RNA. This targeting strategy for m⁶A modification of cytoplasmic RNA viruses is likely relevant for other m⁶A-modified positive-strand RNA viruses with cytoplasmic life cycles such as enterovirus 71 and SARS-CoV-2 and provides an exciting new target for potential antiviral therapies.

Advances in multi-omics research on viral hepatitis

Viral hepatitis is a major global public health problem that affects hundreds of millions of people and is associated with significant morbidity and mortality. Five biologically unrelated hepatotropic viruses account for the majority of the global burden of viral hepatitis, including hepatitis A virus (HAV), hepatitis B virus (HBV), hepatitis C virus (HCV), hepatitis D virus (HDV), and hepatitis E virus (HEV). Omics is defined as the comprehensive study of the functions, relationships and roles of various types of molecules in biological cells. The multi-omics analysis has been proposed and considered key to advancing clinical precision medicine, mainly including genomics, transcriptomics and proteomics, metabolomics. Overall, the applications of multi-omics can show the origin of hepatitis viruses, explore the diagnostic and prognostics biomarkers and screen out the therapeutic targets for viral hepatitis and related diseases. To better understand the pathogenesis of viral hepatitis and related diseases, comprehensive multi-omics analysis has been widely carried out. This review mainly summarizes the applications of multi-omics in different types of viral hepatitis and related diseases, aiming to provide new insight into these diseases.

The application of mass spectrometry method for the study and identification of medically important viruses (review of literature)

It is difficult to overestimate the urgency of the problem of well-timed diagnosis of viral infections. According to the WHO, dozens of outbreaks of viral diseases are recorded annually, both in developing and developed countries. Moreover, the seasonal flu virus alone is capable of infecting up to 20% of the population, even in European countries with a high level of medicine. And the annual number of deaths due to viral infections, according to official statistics, exceeds 600 thousand people around the world. That's why the provision of a reliable and fairly rapid diagnosis of viruses, along with subsequent therapy, makes a significant contribution to reducing the incidence of mortality. Despite the fact that PCR-based methods currently remain the most common method for identifying viruses in clinical practice, as recent experience shows, in addition to the already known disadvantages, in the event of large outbreaks, such test systems may simply not be in the required amount. In this regard, it is necessary to supplement and improve the existing tools for identification and research of clinically significant viruses. The MALDI-TOF mass spectrometry method combines a degree of accuracy and versatility, sufficient both for the identification of clinical strains isolated from patients, and for the study of the phenotypic properties of viruses in research laboratories and centers. This article presents and summarizes the main data on the existing or potential application of the method of time-of-flight mass spectrometry with matrix-associated laser desorption / ionization for the identification or study of viruses.

Generation of Reporter-Expressing New World Arenaviruses: A Systematic Comparison

Replication-competent reporter-expressing viruses are crucial tools in molecular virology with applications that range from antiviral screening to live-cell imaging of protein spatiotemporal dynamics. However, there is currently little information available regarding viable strategies to develop reporter-expressing arenaviruses. To address this, we used Tacaribe virus (TCRV), an apathogenic BSL2 arenavirus, to assess the feasibility of different reporter expression approaches. We first generated trisegmented TCRV viruses with either the glycoprotein (GP) or nucleoprotein (NP) replaced by a reporter (GFP, mCherry, or nanoluciferase). These viruses were all viable, but showed marked differences in brightness and attenuation. Next, we generated terminal fusions with each of the TCRV proteins (i.e., NP, GP, polymerase (L), matrix protein (Z)) either with or without a T2A self-cleavage site. We tested both the function of the reporter-fused proteins alone, and the viability of corresponding recombinant TCRVs. We successfully rescued viruses with both direct and cleavable reporter fusions at the C-terminus of Z, as well as cleavable N-terminal fusions with NP. These viruses all displayed detectable reporter activity, but were also moderately attenuated. Finally, reporter proteins were inserted into a flexible hinge region within L. These viruses were also viable and showed moderate attenuation; however, reporter expression was only detectable for the luminescent virus. These strategies provide an exciting range of new tools for research into the molecular biology of TCRV that can likely also be adapted to other arenaviruses.

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.

WTAP targets the METTL3 m 6 A-methyltransferase complex to cytoplasmic hepatitis C virus RNA to regulate infection

Modification of the hepatitis C virus (HCV) positive-strand RNA genome by N6-methyladenosine (m ⁶ A) regulates the viral lifecycle. This lifecycle takes place solely in the cytoplasm, while m ⁶ A addition on cellular mRNA takes place in the nucleus. Thus, the mechanisms by which m ⁶ A is deposited on the viral RNA have been unclear. In this work, we find that m ⁶ A modification of HCV RNA by the m ⁶ A-methyltransferase proteins METTL3 and METTL14 is regulated by WTAP. WTAP, a predominantly nuclear protein, is an essential member of the cellular mRNA m ⁶ A-methyltransferase complex and known to target METTL3 to mRNA. We found that HCV infection induces localization of WTAP to the cytoplasm. Importantly, we found that WTAP is required for both METTL3 interaction with HCV RNA and for m ⁶ A modification across the viral RNA genome. Further, we found that WTAP, like METTL3 and METTL14, negatively regulates the production of infectious HCV virions, a process that we have previously shown is regulated by m ⁶ A. Excitingly, WTAP regulation of both HCV RNA m ⁶ A modification and virion production were independent of its ability to localize to the nucleus. Together, these results reveal that WTAP is critical for HCV RNA m ⁶ A modification by METTL3 and METTL14 in the cytoplasm.

IMPORTANCE

Positive-strand RNA viruses such as HCV represent a significant global health burden. Previous work has described how HCV RNA contains the RNA modification m ⁶ A and how this modification regulates viral infection. Yet, how this modification is targeted to HCV RNA has remained unclear due to the incompatibility of the nuclear cellular processes that drive m ⁶ A modification with the cytoplasmic HCV lifecycle. In this study, we present evidence for how m ⁶ A modification is targeted to HCV RNA in the cytoplasm by a mechanism in which WTAP recruits the m ⁶ A-methyltransferase METTL3 to HCV RNA. This targeting strategy for m ⁶ A modification of cytoplasmic RNA viruses is likely relevant for other m ⁶ A-modified positive-strand RNA viruses with cytoplasmic lifecycles such as enterovirus 71 and SARS-CoV-2 and provides an exciting new target for potential antiviral therapies.

From systems to structure - using genetic data to model protein structures

Understanding the effects of genetic variation is a fundamental problem in biology that requires methods to analyse both physical and functional consequences of sequence changes at systems-wide and mechanistic scales. To achieve a systems view, protein interaction networks map which proteins physically interact, while genetic interaction networks inform on the phenotypic consequences of perturbing these protein interactions. Until recently, understanding the molecular mechanisms that underlie these interactions often required biophysical methods to determine the structures of the proteins involved. The past decade has seen the emergence of new approaches based on coevolution, deep mutational scanning and genome-scale genetic or chemical-genetic interaction mapping that enable modelling of the structures of individual proteins or protein complexes. Here, we review the emerging use of large-scale genetic datasets and deep learning approaches to model protein structures and their interactions, and discuss the integration of structural data from different sources.

Systems Biology of Virus-Host Protein Interactions: From Hypothesis Generation to Mechanisms of Replication and Pathogenesis

As obligate intracellular parasites, all viruses must co-opt cellular machinery to facilitate their own replication. Viruses often co-opt these cellular pathways and processes through physical interactions between viral and host proteins. In addition to facilitating fundamental aspects of virus replication cycles, these virus-host protein interactions can also disrupt physiological functions of host proteins, causing disease that can be advantageous to the virus or simply a coincidence. Consequently, unraveling virus-host protein interactions can serve as a window into molecular mechanisms of virus replication and pathogenesis. Identifying virus-host protein interactions using unbiased systems biology approaches provides an avenue for hypothesis generation. This review highlights common systems biology approaches for identification of virus-host protein interactions and the mechanistic insights revealed by these methods. We also review conceptual innovations using comparative and integrative systems biology that can leverage global virus-host protein interaction data sets to more rapidly move from hypothesis generation to mechanism. Expected final online publication date for the Annual Review of Virology, Volume 9 is September 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

VAP Proteins - From Organelle Tethers to Pathogenic Host Interactors and Their Role in Neuronal Disease

Vesicle-associated membrane protein (VAMP)-associated proteins (VAPs) are ubiquitous ER-resident tail-anchored membrane proteins in eukaryotic cells. Their N-terminal major sperm protein (MSP) domain faces the cytosol and allows them to interact with a wide variety of cellular proteins. Therefore, VAP proteins are vital to many cellular processes, including organelle membrane tethering, lipid transfer, autophagy, ion homeostasis and viral defence. Here, we provide a timely overview of the increasing number of VAPA/B binding partners and discuss the role of VAPA/B in maintaining organelle-ER interactions and cooperation. Furthermore, we address how viruses and intracellular bacteria hijack VAPs and their binding partners to induce interactions between the host ER and pathogen-containing compartments and support pathogen replication. Finally, we focus on the role of VAP in human disease and discuss how mutated VAPB leads to the disruption of cellular homeostasis and causes amyotrophic lateral sclerosis.

Rotaviral nonstructural protein 5 (NSP5) promotes proteasomal degradation of up-frameshift protein 1 (UPF1), a principal mediator of nonsense-mediated mRNA decay (NMD) pathway, to facilitate infection

Nonsense-mediated mRNA decay (NMD), a cellular RNA quality system, has been shown to be an ancestral form of cellular antiviral response that can restrict viral infection by targeting viral RNA for degradation or other various mechanisms. In support to this hypothesis, emerging evidences unraveled that viruses have evolved numerous mechanisms to circumvent or modulate the NMD pathway to ensure unhindered replication within the host cell. In this study, we investigated the potential interplay between the cellular NMD pathway and rotavirus (RV). Our data suggested that rotavirus infection resulted in global inhibition of NMD pathway by downregulating the expression of UPF1 in a strain independent manner. UPF1 expression was found to be regulated at the post-transcriptional level by ubiquitin-proteasome mediated degradation pathway. Subsequent studies revealed rotaviral non-structural protein 5 (NSP5) associates with UPF1 and promotes its cullin-dependent proteasome mediated degradation. Furthermore, ectopic expression of UPF1 during RV infection resulted in reduced expression of viral proteins and viral RNAs leading to diminished production of infective rotavirus particles, suggesting the anti-rotaviral role of UPF1. Finally, the delayed degradation kinetics of transfected rotaviral RNA in UPF1 and UPF2 depleted cells and the association of UPF1 and UPF2 with viral RNAs suggested that NMD targets rotaviral RNAs for degradation. Collectively, the present study demonstrates the antiviral role of NMD pathway during rotavirus infection and also reveals the underlying mechanism by which rotavirus overwhelms NMD pathway to establish successful replication.

The Link between VAPB Loss of Function and Amyotrophic Lateral Sclerosis

The VAP proteins are integral adaptor proteins of the endoplasmic reticulum (ER) membrane that recruit a myriad of interacting partners to the ER surface. Through these interactions, the VAPs mediate a large number of processes, notably the generation of membrane contact sites between the ER and essentially all other cellular membranes. In 2004, it was discovered that a mutation (p.P56S) in the VAPB paralogue causes a rare form of dominantly inherited familial amyotrophic lateral sclerosis (ALS8). The mutant protein is aggregation-prone, non-functional and unstable, and its expression from a single allele appears to be insufficient to support toxic gain-of-function effects within motor neurons. Instead, loss-of-function of the single wild-type allele is required for pathological effects, and VAPB haploinsufficiency may be the main driver of the disease. In this article, we review the studies on the effects of VAPB deficit in cellular and animal models. Several basic cell physiological processes are affected by downregulation or complete depletion of VAPB, impinging on phosphoinositide homeostasis, Ca2+ signalling, ion transport, neurite extension, and ER stress. In the future, the distinction between the roles of the two VAP paralogues (A and B), as well as studies on motor neurons generated from induced pluripotent stem cells (iPSC) of ALS8 patients will further elucidate the pathogenic basis of p.P56S familial ALS, as well as of other more common forms of the disease.

Structure and function of capsid protein in flavivirus infection and its applications in the development of vaccines and therapeutics

Flaviviruses are enveloped single positive-stranded RNA viruses. The capsid (C), a structural protein of flavivirus, is dimeric and alpha-helical, with several special structural and functional features. The functions of the C protein go far beyond a structural role in virions. It is not only responsible for encapsidation to protect the viral RNA but also able to interact with various host proteins to promote virus proliferation. Therefore, the C protein plays an important role in infected host cells and the viral life cycle. Flaviviruses have been shown to affect the health of humans and animals. Thus, there is an urgent need to effectively control flavivirus infections. The structure of the flavivirus virion has been determined, but there is relatively little information about the function of the C protein. Hence, a greater understanding of the role of the C protein in viral infections will help to discover novel antiviral strategies and provide a promising starting point for the further development of flavivirus vaccines or therapeutics.

Characterisation of the Semliki Forest Virus-host cell interactome reveals the viral capsid protein as an inhibitor of nonsense-mediated mRNA decay

The positive-sense, single-stranded RNA alphaviruses pose a potential epidemic threat. Understanding the complex interactions between the viral and the host cell proteins is crucial for elucidating the mechanisms underlying successful virus replication strategies and for developing specific antiviral interventions. Here we present the first comprehensive protein-protein interaction map between the proteins of Semliki Forest Virus (SFV), a mosquito-borne member of the alphaviruses, and host cell proteins. Among the many identified cellular interactors of SFV proteins, the enrichment of factors involved in translation and nonsense-mediated mRNA decay (NMD) was striking, reflecting the virus' hijacking of the translation machinery and indicating viral countermeasures for escaping NMD by inhibiting NMD at later time points during the infectious cycle. In addition to observing a general inhibition of NMD about 4 hours post infection, we also demonstrate that transient expression of the SFV capsid protein is sufficient to inhibit NMD in cells, suggesting that the massive production of capsid protein during the SFV reproduction cycle is responsible for NMD inhibition.

Hepatitis C virus infection restricts human LINE-1 retrotransposition in hepatoma cells

LINE-1 (L1) retrotransposons are autonomous transposable elements that can affect gene expression and genome integrity. Potential consequences of exogenous viral infections for L1 activity have not been studied to date. Here, we report that hepatitis C virus (HCV) infection causes a significant increase of endogenous L1-encoded ORF1 protein (L1ORF1p) levels and translocation of L1ORF1p to HCV assembly sites at lipid droplets. HCV replication interferes with retrotransposition of engineered L1 reporter elements, which correlates with HCV RNA-induced formation of stress granules and can be partially rescued by knockdown of the stress granule protein G3BP1. Upon HCV infection, L1ORF1p localizes to stress granules, associates with HCV core in an RNA-dependent manner and translocates to lipid droplets. While HCV infection has a negative effect on L1 mobilization, L1ORF1p neither restricts nor promotes HCV infection. In summary, our data demonstrate that HCV infection causes an increase of endogenous L1 protein levels and that the observed restriction of retrotransposition of engineered L1 reporter elements is caused by sequestration of L1ORF1p in HCV-induced stress granules.

Members of the Long Interspersed Nuclear Element 1 (LINE-1, L1) class of retrotransposons account for ~17% of the human genome and include ~100–150 intact L1 loci that are still functional. L1 mobilization is known to affect genomic integrity, thereby leading to disease-causing mutations, but little is known about the impact of exogenous viral infections on L1 and vice versa. While L1 retrotransposition is controlled by various mechanisms including CpG methylation, hypomethylation of L1 has been observed in hepatocellular carcinoma tissues of hepatitis C virus (HCV)-infected patients. Here, we demonstrate molecular interactions between HCV and L1 elements. HCV infection stably increases cellular levels of the L1-encoded ORF1 protein (L1ORF1p). HCV core and L1ORF1p interact in ribonucleoprotein complexes that traffic to lipid droplets. Despite its redistribution to HCV assembly sites, L1ORF1p is dispensable for HCV infection. In contrast, retrotransposition of engineered L1 reporter elements is restricted by HCV, correlating with an increased formation of L1ORF1p-containing cytoplasmic stress granules. Thus, our data provide first insights into the molecular interplay of endogenous transposable elements and exogenous viruses that might contribute to disease progression in vivo.

Nonsense-mediated mRNA decay does not restrict influenza A virus propagation

Nonsense-mediated mRNA decay (NMD) was identified as a process to degrade flawed cellular messenger RNA (mRNA). Within the last decades it was also shown that NMD carries virus-restricting capacities and thus could be considered a part of the cellular antiviral system. As this was shown to affect primarily positive-sense single stranded RNA ((+)ssRNA) viruses there is only scarce knowledge if this also applies to negative-sense single stranded RNA ((-)ssRNA) viruses. Influenza A viruses (IAVs) harbour a segmented (-)ssRNA genome. During their replication IAVs produce numerous RNA transcripts and simultaneously impair cellular transcription and translation. The viral mRNAs hold several molecular patterns which can elicit NMD and in turn would lead to their degradation. This, in consequence, may mitigate viral propagation. Thus, we examined if a knockdown or a pharmacological inhibition of NMD key components may influence IAV replication. Additionally, we performed similar experiments with respiratory syncytial virus (RSV), another (-)ssRNA virus, but with a non-segmented genome. Although it seemed that a knockdown of up-frameshift protein 1 (UPF1), the central NMD factor, slightly increased viral mRNA and protein levels, no significant alteration of viral replication could be observed, implying that the NMD machinery may not have restricting capacities against (-)ssRNA viruses.

Current status and future perspectives of computational studies on human-virus protein-protein interactions

The protein-protein interactions (PPIs) between human and viruses mediate viral infection and host immunity processes. Therefore, the study of human-virus PPIs can help us understand the principles of human-virus relationships and can thus guide the development of highly effective drugs to break the transmission of viral infectious diseases. Recent years have witnessed the rapid accumulation of experimentally identified human-virus PPI data, which provides an unprecedented opportunity for bioinformatics studies revolving around human-virus PPIs. In this article, we provide a comprehensive overview of computational studies on human-virus PPIs, especially focusing on the method development for human-virus PPI predictions. We briefly introduce the experimental detection methods and existing database resources of human-virus PPIs, and then discuss the research progress in the development of computational prediction methods. In particular, we elaborate the machine learning-based prediction methods and highlight the need to embrace state-of-the-art deep-learning algorithms and new feature engineering techniques (e.g. the protein embedding technique derived from natural language processing). To further advance the understanding in this research topic, we also outline the practical applications of the human-virus interactome in fundamental biological discovery and new antiviral therapy development.

Nonsense-mediated decay controls the reactivation of the oncogenic herpesviruses EBV and KSHV

The oncogenic human herpesviruses Epstein-Barr virus (EBV) and Kaposi's sarcoma-associated herpesvirus (KSHV) are the causative agents of multiple malignancies. A hallmark of herpesviruses is their biphasic life cycle consisting of latent and lytic infection. In this study, we identified that cellular nonsense-mediated decay (NMD), an evolutionarily conserved RNA degradation pathway, critically regulates the latent-to-lytic switch of EBV and KSHV infection. The NMD machinery suppresses EBV and KSHV Rta transactivator expression and promotes maintenance of viral latency by targeting the viral polycistronic transactivator transcripts for degradation through the recognition of features in their 3' UTRs. Treatment with a small-molecule NMD inhibitor potently induced reactivation in a variety of EBV- and KSHV-infected cell types. In conclusion, our results identify NMD as an important host process that controls oncogenic herpesvirus reactivation, which may be targeted for the therapeutic induction of lytic reactivation and the eradication of tumor cells.

Mass spectrometry-based protein‒protein interaction techniques and their applications in studies of DNA damage repair

Proteins are major functional units that are tightly connected to form complex and dynamic networks. These networks enable cells and organisms to operate properly and respond efficiently to environmental cues. Over the past decades, many biochemical methods have been developed to search for protein-binding partners in order to understand how protein networks are constructed and connected. At the same time, rapid development in proteomics and mass spectrometry (MS) techniques makes it possible to identify interacting proteins and build comprehensive protein‒protein interaction networks. The resulting interactomes and networks have proven informative in the investigation of biological functions, such as in the field of DNA damage repair. In recent years, a number of proteins involved in DNA damage response and DNA repair pathways have been uncovered with MS-based protein‒protein interaction studies. As the technologies for enriching associated proteins and MS become more sophisticated, the studies of protein‒protein interactions are entering a new era. In this review, we summarize the strategies and recent developments for exploring protein‒protein interaction. In addition, we discuss the application of these tools in the investigation of protein‒protein interaction networks involved in DNA damage response and DNA repair.

Invisible leashes: The tethering VAPs from infectious diseases to neurodegeneration

Intracellular organelles do not, as thought for a long time, act in isolation but are dynamically tethered together by entire machines responsible for interorganelle trafficking and positioning. Among the proteins responsible for tethering is the family of VAMP-associated proteins (VAPs) that appear in all eukaryotes and are localized primarily in the endoplasmic reticulum. The major functional role of VAPs is to tether the endoplasmic reticulum with different organelles and regulate lipid metabolism and transport. VAPs have gained increasing attention because of their role in human pathology where they contribute to infections by viruses and bacteria and participate in neurodegeneration. In this review, we discuss the structure, evolution, and functions of VAPs, focusing more specifically on VAP-B for its relationship with amyotrophic lateral sclerosis and other neurodegenerative diseases.

Mass spectrometry-based protein-protein interaction networks for the study of human diseases

A better understanding of the molecular mechanisms underlying disease is key for expediting the development of novel therapeutic interventions. Disease mechanisms are often mediated by interactions between proteins. Insights into the physical rewiring of protein-protein interactions in response to mutations, pathological conditions, or pathogen infection can advance our understanding of disease etiology, progression, and pathogenesis and can lead to the identification of potential druggable targets. Advances in quantitative mass spectrometry (MS)-based approaches have allowed unbiased mapping of these disease-mediated changes in protein-protein interactions on a global scale. Here, we review MS techniques that have been instrumental for the identification of protein-protein interactions at a system-level, and we discuss the challenges associated with these methodologies as well as novel MS advancements that aim to address these challenges. An overview of examples from diverse disease contexts illustrates the potential of MS-based protein-protein interaction mapping approaches for revealing disease mechanisms, pinpointing new therapeutic targets, and eventually moving toward personalized applications.

Targeting of viral RNAs by Upf1-mediated RNA decay pathways

Viral RNAs are susceptible to co-translational RNA decay pathways mediated by the RNA helicase Upstream frameshift 1 (Upf1). Upf1 is a key component in nonsense-mediated decay (NMD), Staufen1-mediated mRNA decay (SMD), and structure-mediated RNA decay (SRD) pathways, among others. Diverse families of viruses have features that predispose them to Upf1 targeting, but have evolved means to escape decay through the action of cis-acting or trans-acting viral factors. Studies aimed at understanding how viruses are subjected to and circumvent NMD have increased our understanding of NMD target selection of host mRNAs. This review focuses on the knowledge gained from studying NMD in viral systems as well as related Upf1-dependent pathways and how these pathways restrict virus replication.

SARS-CoV-2-Encoded Proteome and Human Genetics: From Interaction-Based to Ribosomal Biology Impact on Disease and Risk Processes

SARS-CoV-2 (COVID-19) has infected millions of people worldwide, with lethality in hundreds of thousands. The rapid publication of information, both regarding the clinical course and the viral biology, has yielded incredible knowledge of the virus. In this review, we address the insights gained for the SARS-CoV-2 proteome, which we have integrated into the Viral Integrated Structural Evolution Dynamic Database, a publicly available resource. Integrating evolutionary, structural, and interaction data with human proteins, we present how the SARS-CoV-2 proteome interacts with human disorders and risk factors ranging from cytokine storm, hyperferritinemic septic, coagulopathic, cardiac, immune, and rare disease-based genetics. The most noteworthy human genetic potential of SARS-CoV-2 is that of the nucleocapsid protein, where it is known to contribute to the inhibition of the biological process known as nonsense-mediated decay. This inhibition has the potential to not only regulate about 10% of all biological transcripts through altered ribosomal biology but also associate with viral-induced genetics, where suppressed human variants are activated to drive dominant, negative outcomes within cells. As we understand more of the dynamic and complex biological pathways that the proteome of SARS-CoV-2 utilizes for entry into cells, for replication, and for release from human cells, we can understand more risk factors for severe/lethal outcomes in patients and novel pharmaceutical interventions that may mitigate future pandemics.

Viral subversion of nonsense-mediated mRNA decay

Viruses have evolved in tandem with the organisms that they infect. Afflictions of the plant and animal kingdoms with viral infections have forced the host organism to evolve new or exploit existing systems to develop the countermeasures needed to offset viral insults. As one example, nonsense-mediated mRNA decay, a cellular quality-control mechanism ensuring the translational fidelity of mRNA transcripts, has been used to restrict virus replication in both plants and animals. In response, viruses have developed a slew of means to disrupt or become insensitive to NMD, providing researchers with potential new reagents that can be used to more fully understand the NMD mechanism.

Coronavirus: proteomics analysis of chicken kidney tissue infected with variant 2 (IS-1494)-like avian infectious bronchitis virus

Avian infectious bronchitis virus is one of the most important gammacoronaviruses, which causes a highly contagious disease. In this study, we investigated changes in the proteome of kidney tissue of specific-pathogen-free (SPF) chickens that were infected with an isolate of the nephrotropic variant 2 genotype (IS/1494/06) of avian coronavirus. Twenty 1-day-old SPF White Leghorn chickens were randomly divided into two groups, each comprising 10 chickens, which were kept in separate positive-pressure isolators. Chickens in group A served as a virus-free control group up to the end of the experiment, whereas chickens in group B were inoculated with 0.1 ml of 10^4.5 EID₅₀ of the IBV/chicken/Iran/UTIVO-C/2014 isolate of IBV, and kidney tissue samples were collected at 2 and 7 days post-inoculation (dpi) from both groups. Sequencing of five protein spots at 2 dpi and 22 spots at 7 dpi that showed differential expression by two-dimensional electrophoresis (2DE) along with fold change greater than 2 was done by MS-MALDI/TOF/TOF. Furthermore, the corresponding protein-protein interaction (PPI) networks at 2 and 7 dpi were identified to develop a detailed understanding of the mechanism of molecular pathogenesis. Topological graph analysis of this undirected PPI network revealed the effect of 10 genes in the 2 dpi PPI network and nine genes in the 7 dpi PPI network during virus pathogenesis. Proteins that were found by 2DE analysis and MS/TOF-TOF mass spectrometry to be down- or upregulated were subjected to PPI network analysis to identify interactions with other cellular components. The results show that cellular metabolism was altered due to viral infection. Additionally, multifunctional heat shock proteins with a significant role in host cell survival may be employed circuitously by the virus to reach its target. The data from this study suggest that the process of pathogenesis that occurs during avian coronavirus infection involves the regulation of vital cellular processes and the gradual disruption of critical cellular functions.

Mitochondria Targeted Viral Replication and Survival Strategies-Prospective on SARS-CoV-2

SARS-CoV-2 is a positive sense RNA coronavirus that constitutes a new threat for the global community and economy. While vaccines against SARS-CoV-2 are being developed, the mechanisms through which this virus takes control of an infected cell to replicate remains poorly understood. Upon infection, viruses completely rely on host cell molecular machinery to survive and replicate. To escape from the immune response and proliferate, viruses strategically modulate cellular metabolism and alter subcellular organelle architecture and functions. One way they do this is by modulating the structure and function of mitochondria, a critical cellular metabolic hub but also a key platform for the regulation of cellular immunity. This versatile nature of mitochondria defends host cells from viruses through several mechanisms including cellular apoptosis, ROS signaling, MAVS activation and mitochondrial DNA-dependent immune activation. These events are regulated by mitochondrial dynamics, a process by which mitochondria alter their structure (including their length and connectivity) in response to stress or other cues. It is therefore not surprising that viruses, including coronaviruses hijack these processes for their survival. In this review, we highlight how positive sense RNA viruses modulate mitochondrial dynamics and metabolism to evade mitochondrial mediated immune response in order to proliferate.

Host-derived circular RNAs display proviral activities in Hepatitis C virus-infected cells

Viruses subvert macromolecular pathways in infected host cells to aid in viral gene amplification or to counteract innate immune responses. Roles for host-encoded, noncoding RNAs, including microRNAs, have been found to provide pro- and anti-viral functions. Recently, circular RNAs (circRNAs), that are generated by a nuclear back-splicing mechanism of pre-mRNAs, have been implicated to have roles in DNA virus-infected cells. This study examines the circular RNA landscape in uninfected and hepatitis C virus (HCV)-infected liver cells. Results showed that the abundances of distinct classes of circRNAs were up-regulated or down-regulated in infected cells. Identified circRNAs displayed pro-viral effects. One particular up-regulated circRNA, circPSD3, displayed a very pronounced effect on viral RNA abundances in both hepatitis C virus- and Dengue virus-infected cells. Though circPSD3 has been shown to bind factor eIF4A3 that modulates the cellular nonsense-mediated decay (NMD) pathway, circPSD3 regulates RNA amplification in a pro-viral manner at a post-translational step, while eIF4A3 exhibits the anti-viral property of the NMD pathway. Findings from the global analyses of the circular RNA landscape argue that pro-, and likely, anti-viral functions are executed by circRNAs that modulate viral gene expression as well as host pathways. Because of their long half-lives, circRNAs likely play hitherto unknown, important roles in viral pathogenesis.