Publications

2023

Mathematical models have played a crucial role in exploring and guiding pandemic responses. University campuses present a particularly well-documented case for institutional outbreaks, thereby providing a unique opportunity to understand detailed patterns of pathogen spread. Here, we present descriptive and modeling analyses of SARS-CoV-2 transmission on the Princeton University (PU) campus-this model was used throughout the pandemic to inform policy decisions and operational guidelines for the university campus. Epidemic patterns between the university campus and surrounding communities exhibit strong spatiotemporal correlations. Mathematical modeling analysis further suggests that the amount of on-campus transmission was likely limited during much of the wider pandemic until the end of 2021. Finally, we find that a superspreading event likely played a major role in driving the Omicron variant outbreak on the PU campus during the spring semester of the 2021-2022 academic year. Despite large numbers of cases on campus in this period, case levels in surrounding communities remained low, suggesting that there was little spillover transmission from campus to the local community.

Targeted protein degradation (TPD), as exemplified by proteolysis-targeting chimera (PROTAC), is an emerging drug discovery platform. PROTAC molecules, which typically contain a target protein ligand linked to an E3 ligase ligand, recruit a target protein to the E3 ligase to induce its ubiquitination and degradation. Here, we applied PROTAC approaches to develop broad-spectrum antivirals targeting key host factors for many viruses and virus-specific antivirals targeting unique viral proteins. For host-directed antivirals, we identified a small-molecule degrader, FM-74-103, that elicits selective degradation of human GSPT1, a translation termination factor. FM-74-103-mediated GSPT1 degradation inhibits both RNA and DNA viruses. Among virus-specific antivirals, we developed viral RNA oligonucleotide-based bifunctional molecules (Destroyers). As a proof of principle, RNA mimics of viral promoter sequences were used as heterobifunctional molecules to recruit and target influenza viral polymerase for degradation. This work highlights the broad utility of TPD to rationally design and develop next-generation antivirals.

The influenza A virus (IAV) RNA polymerase is an essential driver of IAV evolution. Mutations that the polymerase introduces into viral genome segments during replication are the ultimate source of genetic variation, including within the three subunits of the IAV polymerase (polymerase basic protein 2, polymerase basic protein 1, and polymerase acidic protein). Evolutionary analysis of the IAV polymerase is complicated, because changes in mutation rate, replication speed, and drug resistance involve epistatic interactions among its subunits. In order to study the evolution of the human seasonal H3N2 polymerase since the 1968 pandemic, we identified pairwise evolutionary relationships among ∼7000 H3N2 polymerase sequences using mutual information (MI), which measures the information gained about the identity of one residue when a second residue is known. To account for uneven sampling of viral sequences over time, we developed a weighted MI (wMI) metric and demonstrate that wMI outperforms raw MI through simulations using a well-sampled severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) dataset. We then constructed wMI networks of the H3N2 polymerase to extend the inherently pairwise wMI statistic to encompass relationships among larger groups of residues. We included hemagglutinin (HA) in the wMI network to distinguish between functional wMI relationships within the polymerase and those potentially due to hitch-hiking on antigenic changes in HA. The wMI networks reveal coevolutionary relationships among residues with roles in replication and encapsidation. Inclusion of HA highlighted polymerase-only subgraphs containing residues with roles in the enzymatic functions of the polymerase and host adaptability. This work provides insight into the factors that drive and constrain the rapid evolution of influenza viruses.

Siegers, Jurre Y et al. “Evolution of Highly Pathogenic H5N1 Influenza A Virus in the Central Nervous System of Ferrets..” PLoS pathogens 19.3 (2023): e1011214.

Central nervous system (CNS) disease is the most common extra-respiratory tract complication of influenza A virus infections in humans. Remarkably, zoonotic highly pathogenic avian influenza (HPAI) H5N1 virus infections are more often associated with CNS disease than infections with seasonal influenza viruses. Evolution of avian influenza viruses has been extensively studied in the context of respiratory infections, but evolutionary processes in CNS infections remain poorly understood. We have previously observed that the ability of HPAI A/Indonesia/5/2005 (H5N1) virus to replicate in and spread throughout the CNS varies widely between individual ferrets. Based on these observations, we sought to understand the impact of entrance into and replication within the CNS on the evolutionary dynamics of virus populations. First, we identified and characterized three substitutions-PB1 E177G and A652T and NP I119M - detected in the CNS of a ferret infected with influenza A/Indonesia/5/2005 (H5N1) virus that developed a severe meningo-encephalitis. We found that some of these substitutions, individually or collectively, resulted in increased polymerase activity in vitro. Nevertheless, in vivo, the virus bearing the CNS-associated mutations retained its capacity to infect the CNS but showed reduced dispersion to other anatomical sites. Analyses of viral diversity in the nasal turbinate and olfactory bulb revealed the lack of a genetic bottleneck acting on virus populations accessing the CNS via this route. Furthermore, virus populations bearing the CNS-associated mutations showed signs of positive selection in the brainstem. These features of dispersion to the CNS are consistent with the action of selective processes, underlining the potential for H5N1 viruses to adapt to the CNS.

2022

During infection, the influenza A virus RNA polymerase produces both full-length and aberrant RNA molecules, such as defective viral genomes (DVGs) and mini viral RNAs (mvRNAs). Subsequent innate immune activation involves the binding of host pathogen receptor retinoic acid-inducible gene I (RIG-I) to viral RNAs. However, it is not clear what factors determine which influenza A virus RNAs are RIG-I agonists. Here, we provide evidence that RNA structures, called template loops (t-loops), stall the viral RNA polymerase and contribute to innate immune activation by mvRNAs during influenza A virus infection. Impairment of replication by t-loops depends on the formation of an RNA duplex near the template entry and exit channels of the RNA polymerase, and this effect is enhanced by mutation of the template exit path from the RNA polymerase active site. Overall, these findings are suggestive of a mechanism involving polymerase stalling that links aberrant viral replication to the activation of the innate immune response.
Bisht, Karishma, and Aartjan Te Velthuis. “Decoding the Role of Temperature in RNA Virus Infections.” mBio (2022): e0202122.
RNA viruses include respiratory viruses, such as coronaviruses and influenza viruses, as well as vector-borne viruses, like dengue and West Nile virus. RNA viruses like these encounter various environments when they copy themselves and spread from cell to cell or host to host. Ex vivo differences, such as geographical location and humidity, affect their stability and transmission, while in vivo differences, such as pH and host gene expression, impact viral receptor binding, viral replication, and the host immune response against the viral infection. A critical factor affecting RNA viruses both ex vivo and in vivo, and defining the outcome of viral infections and the direction of viral evolution, is temperature. In this minireview, we discuss the impact of temperature on viral replication, stability, transmission, and adaptation, as well as the host innate immune response. Improving our understanding of how RNA viruses function, survive, and spread at different temperatures will improve our models of viral replication and transmission risk analyses.
Pitre, Emmanuelle, and Aartjan Te Velthuis. “Tinker, Tailor, Antiviral: RNA Virus Inhibition by Induced Recombination.” Trends Biochem Sci (2022): n. pag.
Nucleotide analogs can help to combat RNA virus growth by stalling the viral RNA polymerase or by introducing lethal mutations into the viral genome. Janissen and Woodman et al. have used single-molecule, sequencing, and virological methods to reveal that antiviral T-1106 provides a third mechanism of counterattack: inducing recombination.

2021

Nilsson-Payant, Benjamin, , and Aartjan Te Velthuis. “The Host Factor ANP32A Is Required for Influenza A Virus VRNA and CRNA Synthesis.” J Virol (2021): jvi0209221.
Influenza A viruses are negative-sense RNA viruses that rely on their own viral replication machinery to replicate and transcribe their segmented single-stranded RNA genome. The viral ribonucleoprotein complexes in which viral RNA is replicated consist of a nucleoprotein scaffold around which the RNA genome is bound, and a heterotrimeric RNA-dependent RNA polymerase that catalyzes viral replication. The RNA polymerase copies the viral RNA (vRNA) via a replicative intermediate, called the complementary RNA (cRNA), and subsequently uses this cRNA to make more vRNA copies. To ensure that new cRNA and vRNA molecules are associated with ribonucleoproteins in which they can be amplified, the active RNA polymerase recruits a second polymerase to encapsidate the cRNA or vRNA. Host factor ANP32A has been shown to be essential for viral replication and to facilitate the formation of a dimer between viral RNA polymerases. Differences between mammalian and avian ANP32A proteins are sufficient to restrict viral replication. It has been proposed that ANP32A is only required for the synthesis of vRNA molecules from a cRNA, but not vice versa. However, this view does not match recent molecular evidence. Here we use minigenome assays, virus infections, and viral promoter mutations to demonstrate that ANP32A is essential for both vRNA and cRNA synthesis. Moreover, we show that ANP32 is not only needed for the actively replicating polymerase, but also for the polymerase that is encapsidating nascent viral RNA products. Overall, these results provide new insights into influenza A virus replication and host adaptation. Zoonotic avian influenza A viruses pose a constant threat to global health, and they have the potential to cause pandemics. Species variations in host factor ANP32A play a key role in supporting the activity of avian influenza A virus RNA polymerases in mammalian hosts. Here we show that ANP32A acts at two stages in the influenza A virus replication cycle, supporting recent structural experiments, in line with its essential role. Understanding how ANP32A supports viral RNA polymerase activity and how it supports avian polymerase function in mammalian hosts is important for understanding influenza A virus replication and the development of antiviral strategies against influenza A viruses.
Deletion-containing viral genomes (DelVGs) are commonly produced during influenza A virus infection and have been implicated in influencing clinical infection outcomes. Despite their ubiquity, the specific molecular mechanisms that govern DelVG formation and their packaging into defective interfering particles (DIPs) remain poorly understood. Here, we utilized next-generation sequencing to analyze DelVGs that form early during infection, prior to packaging. Analysis of these early DelVGs revealed that deletion formation occurs in clearly defined hot spots and is significantly associated with both direct sequence repeats and enrichment of adenosine and uridine bases. By comparing intracellular DelVGs with those packaged into extracellular virions, we discovered that DelVGs face a significant bottleneck during genome packaging relative to wild-type genomic RNAs. Interestingly, packaged DelVGs exhibited signs of enrichment for larger DelVGs suggesting that size is an important determinant of packaging efficiency. Our data provide the first unbiased, high-resolution portrait of the diversity of DelVGs that are generated by the influenza A virus replication machinery and shed light on the mechanisms that underly DelVG formation and packaging. Defective interfering particles (DIPs) are commonly produced by RNA viruses and have been implicated in modulating clinical infection outcomes; hence, there is increasing interest in the potential of DIPs as antiviral therapeutics. For influenza viruses, DIPs are formed by the packaging of genomic RNAs harboring internal deletions. Despite decades of study, the mechanisms that drive the formation of these deletion-containing viral genomes (DelVGs) remain elusive. Here, we used a specialized sequencing pipeline to characterize the first wave of DelVGs that form during influenza virus infection. This data set provides an unbiased profile of the deletion-forming preferences of the influenza virus replicase. In addition, by comparing the early intracellular DelVGs to those that get packaged into extracellular virions, we described a significant segment-specific bottleneck that limits DelVG packaging relative to wild-type viral RNAs. Altogether, these findings reveal factors that govern the production of both DelVGs and DIPs during influenza virus infection.
Pitre, Emmanuelle, and Aartjan Te Velthuis. “Understanding Viral Replication and Transcription Using Single-Molecule Techniques.” Enzymes 49 (2021): 83–113.
DNA and RNA viruses depend on one or more enzymes to copy and transcribe their genome, such as a polymerase, helicase, or exonuclease. Because of the important role of these enzymes in the virus replication cycle, they are key targets for antiviral development. To better understand the function of these enzymes and their interactions with host and viral factors, biochemical, structural and single-molecule approaches have been used to study them. Each of these techniques has its own strengths, and single-molecule methods have proved particularly powerful in providing insight into the step-sizes of motor proteins, heterogeneity of enzymatic activities, transient conformational changes, and force-sensitivity of reactions. Here we will discuss how single-molecule FRET, magnetic tweezers, optical tweezers, atomic force microscopy and flow stretching approaches have revealed novel insights into polymerase fidelity, the mechanism of action of antivirals, and the protein choreography within replication complexes.
Elshina, Elizaveta, and Aartjan Te Velthuis. “The Influenza Virus RNA Polymerase As an Innate Immune Agonist and Antagonist.” Cell Mol Life Sci (2021): n. pag.
Influenza A viruses cause a mild-to-severe respiratory disease that affects millions of people each year. One of the many determinants of disease outcome is the innate immune response to the viral infection. While antiviral responses are essential for viral clearance, excessive innate immune activation promotes lung damage and disease. The influenza A virus RNA polymerase is one of viral proteins that affect innate immune activation during infection, but the mechanisms behind this activity are not well understood. In this review, we discuss how the viral RNA polymerase can both activate and suppress innate immune responses by either producing immunostimulatory RNA species or directly targeting the components of the innate immune signalling pathway, respectively. Furthermore, we provide a comprehensive overview of the polymerase residues, and their mutations, associated with changes in innate immune activation, and discuss their putative effects on polymerase function based on recent advances in our understanding of the influenza A virus RNA polymerase structure.
Elli, Stefano et al. “Enisamium Inhibits SARS-CoV-2 RNA Synthesis.” Biomedicines 9.9 (2021): n. pag.
Pandemic SARS-CoV-2 causes a mild to severe respiratory disease called coronavirus disease 2019 (COVID-19). While control of the SARS-CoV-2 spread partly depends on vaccine-induced or naturally acquired protective herd immunity, antiviral strategies are still needed to manage COVID-19. Enisamium is an inhibitor of influenza A and B viruses in cell culture and clinically approved in countries of the Commonwealth of Independent States. In vitro, enisamium acts through metabolite VR17-04 and inhibits the activity of the influenza A virus RNA polymerase. Here we show that enisamium can inhibit coronavirus infections in NHBE and Caco-2 cells, and the activity of the SARS-CoV-2 RNA polymerase in vitro. Docking and molecular dynamics simulations provide insight into the mechanism of action and indicate that enisamium metabolite VR17-04 prevents GTP and UTP incorporation. Overall, these results suggest that enisamium is an inhibitor of SARS-CoV-2 RNA synthesis in vitro.
Gopal, Vikram et al. “Zinc-Embedded Polyamide Fabrics Inactivate SARS-CoV-2 and Influenza A Virus.” ACS Appl Mater Interfaces 13.26 (2021): 30317–30325.
Influenza A viruses (IAV) and SARS-CoV-2 can spread via liquid droplets and aerosols. Face masks and other personal protective equipment (PPE) can act as barriers that prevent the spread of these viruses. However, IAV and SARS-CoV-2 are stable for hours on various materials, which makes frequent and correct disposal of these PPE important. Metal ions embedded into PPE may inactivate respiratory viruses, but confounding factors such as adsorption of viruses make measuring and optimizing the inactivation characteristics difficult. Here, we used polyamide 6.6 (PA66) fibers containing embedded zinc ions and systematically investigated if these fibers can adsorb and inactivate SARS-CoV-2 and IAV H1N1 when woven into a fabric. We found that our PA66-based fabric decreased the IAV H1N1 and SARS-CoV-2 titer by approximately 100-fold. Moreover, we found that the zinc content and the virus inactivating property of the fabric remained stable over 50 standardized washes. Overall, these results provide insights into the development of reusable PPE that offer protection against RNA virus spread.
Weis, Sabrina, and Aartjan Te Velthuis. “Influenza Virus RNA Synthesis and the Innate Immune Response.” Viruses 13.5 (2021): n. pag.
Infection with influenza A and B viruses results in a mild to severe respiratory tract infection. It is widely accepted that many factors affect the severity of influenza disease, including viral replication, host adaptation, innate immune signalling, pre-existing immunity, and secondary infections. In this review, we will focus on the interplay between influenza virus RNA synthesis and the detection of influenza virus RNA by our innate immune system. Specifically, we will discuss the generation of various RNA species, host pathogen receptors, and host shut-off. In addition, we will also address outstanding questions that currently limit our knowledge of influenza virus replication and host adaption. Understanding the molecular mechanisms underlying these factors is essential for assessing the pandemic potential of future influenza virus outbreaks.
Te Velthuis, Aartjan et al. “Enisamium Reduces Influenza Virus Shedding and Improves Patient Recovery by Inhibiting Viral RNA Polymerase Activity.” Antimicrob Agents Chemother (2021): n. pag.
Infections with respiratory viruses constitute a huge burden on our health and economy. Antivirals against some respiratory viruses are available, but further options are urgently needed. Enisamium iodide (laboratory code FAV00A, trade name Amizon®) is an antiviral marketed in countries of the Commonwealth of Independent States for the treatment of viral respiratory infections, but its clinical efficacy and mode of action are not well understood. Here, we investigated the efficacy of enisamium in patients aged between 18-60 years with confirmed influenza and other viral respiratory infections. Enisamium treatment resulted in reduced influenza virus shedding (at day 3, 71.2% in enisamium group tested negative versus 25.0% in placebo group, p
Te Velthuis, Aartjan, Jonathan Grimes, and Ervin Fodor. “Structural Insights into RNA Polymerases of Negative-Sense RNA Viruses.” Nat Rev Microbiol (2021): n. pag.
RNA viruses include many important human and animal pathogens, such as the influenza viruses, respiratory syncytial virus, Ebola virus, measles virus and rabies virus. The genomes of these viruses consist of single or multiple RNA segments that assemble with oligomeric viral nucleoprotein into ribonucleoprotein complexes. Replication and transcription of the viral genome is performed by ~250-450 kDa viral RNA-dependent RNA polymerases that also contain capping or cap-snatching activity. In this Review, we compare recent high-resolution X-ray and cryoelectron microscopy structures of RNA polymerases of negative-sense RNA viruses with segmented and non-segmented genomes, including orthomyxoviruses, peribunyaviruses, phenuiviruses, arenaviruses, rhabdoviruses, pneumoviruses and paramyxoviruses. In addition, we discuss how structural insights into these enzymes contribute to our understanding of the molecular mechanisms of viral transcription and replication, and how we can use these insights to identify targets for antiviral drug design.
Pandemic SARS-CoV-2 causes a mild to severe respiratory disease called Coronavirus Disease 2019 (COVID-19). Control of SARS-CoV-2 spread will depend on vaccine-induced or naturally acquired protective herd immunity. Until then, antiviral strategies are needed to manage COVID-19, but approved antiviral treatments, such as remdesivir, can only be delivered intravenously. Enisamium (laboratory code FAV00A, trade name Amizon®) is an orally active inhibitor of influenza A and B viruses in cell culture and clinically approved in countries of the Commonwealth of Independent States. Here we show that enisamium can inhibit SARS-CoV-2 infections in NHBE and Caco-2 cells. , the previously identified enisamium metabolite VR17-04 directly inhibits the activity of the SARS-CoV-2 RNA polymerase. Docking and molecular dynamics simulations suggest that VR17-04 prevents GTP and UTP incorporation. To confirm enisamium's antiviral properties, we conducted a double-blind, randomized, placebo-controlled trial in adult, hospitalized COVID-19 patients, which needed medical care either with or without supplementary oxygen. Patients received either enisamium (500 mg per dose) or placebo for 7 days. A pre-planned interim analysis showed in the subgroup of patients needing supplementary oxygen (n = 77) in the enisamium group a mean recovery time of 11.1 days, compared to 13.9 days for the placebo group (log-rank test; p=0.0259). No significant difference was found for all patients (n = 373) or those only needing medical care (n = 296). These results thus suggest that enisamium is an inhibitor of SARS-CoV-2 RNA synthesis and that enisamium treatment shortens the time to recovery for COVID-19 patients needing oxygen. Significance statement: SARS-CoV-2 is the causative agent of COVID-19. Although vaccines are now becoming available to prevent SARS-CoV-2 spread, the development of antivirals remains necessary for treating current COVID-19 patients and combating future coronavirus outbreaks. Here, we report that enisamium, which can be administered orally, can prevent SARS-CoV-2 replication and that its metabolite VR17-04 can inhibit the SARS-CoV-2 RNA polymerase . Moreover, we find that COVID-19 patients requiring supplementary oxygen, recover more quickly than patients treated with a placebo. Enisamium may therefore be an accessible treatment for COVID-19 patients.

2020

Deubiquitylases (DUBs) regulate critical signaling pathways at the intersection of host immunity and viral pathogenesis. Although RIG-I activation is heavily dependent on ubiquitylation, systematic analyses of DUBs that regulate this pathway have not been performed. Using a ubiquitin C-terminal electrophile, we profile DUBs that function during influenza A virus (IAV) infection and isolate OTUB1 as a key regulator of RIG-I-dependent antiviral responses. Upon infection, OTUB1 relocalizes from the nucleus to mitochondrial membranes together with RIG-I, viral PB2, and NS1. Its expression depends on competing effects of interferon stimulation and IAV-triggered degradation. OTUB1 activates RIG-I via a dual mechanism of K48 polyubiquitin hydrolysis and formation of an E2-repressive complex with UBCH5c. We reconstitute this mechanism in a cell-free system comprising [S]IRF3, purified RIG-I, mitochondrial membranes, and cytosol expressing OTUB1 variants. A range of IAV NS1 proteins trigger proteasomal degradation of OTUB1, antagonizing the RIG-I signaling cascade and antiviral responses.
Influenza A virus and coronavirus strains cause a mild to severe respiratory disease that can result in death. Although vaccines exist against circulating influenza A viruses, such vaccines are ineffective against emerging pandemic influenza A viruses. Currently, no vaccine exists against coronavirus infections, including pandemic SARS-CoV-2, the causative agent of the Coronavirus Disease 2019 (COVID-19). To combat these RNA virus infections, alternative antiviral strategies are needed. A key drug target is the viral RNA polymerase, which is responsible for viral RNA synthesis. In January 2020, the World Health Organisation identified enisamium as a candidate therapeutic against SARS-CoV-2. Enisamium is an isonicotinic acid derivative that is an inhibitor of multiple influenza B and A virus strains in cell culture and clinically approved in 11 countries. Here we show using assays that enisamium and its putative metabolite, VR17-04, inhibit the activity of the influenza virus and the SARS-CoV-2 RNA polymerase. VR17-04 displays similar efficacy against the SARS-CoV-2 RNA polymerase as the nucleotide analogue remdesivir triphosphate. These results suggest that enisamium is a broad-spectrum small molecule inhibitor of RNA virus RNA synthesis, and implicate it as a possible therapeutic option for treating SARS-CoV-2 infection. Unlike remdesivir, enisamium does not require intravenous administration which may be advantageous for the development of COVID-19 treatments outside a hospital setting.
Fodor, Ervin, and Aartjan Te Velthuis. “Structure and Function of the Influenza Virus Transcription and Replication Machinery.” Cold Spring Harb Perspect Med 10.9 (2020): n. pag.
Transcription and replication of the influenza virus RNA genome is catalyzed by the viral heterotrimeric RNA-dependent RNA polymerase in the context of viral ribonucleoprotein (vRNP) complexes. Atomic resolution structures of the viral RNA synthesis machinery have offered insights into the initiation mechanisms of viral transcription and genome replication, and the interaction of the viral RNA polymerase with host RNA polymerase II, which is required for the initiation of viral transcription. Replication of the viral RNA genome by the viral RNA polymerase depends on host ANP32A, and host-specific sequence differences in ANP32A underlie the poor activity of avian influenza virus polymerases in mammalian cells. A failure to faithfully copy the viral genome segments can lead to the production of aberrant viral RNA products, such as defective interfering (DI) RNAs and mini viral RNAs (mvRNAs). Both aberrant RNA types have been implicated in innate immune responses against influenza virus infection. This review discusses recent insights into the structure-function relationship of the viral RNA polymerase and its role in determining host range and virulence.

2019

Russell, Alistair et al. “Single-Cell Virus Sequencing of Influenza Infections That Trigger Innate Immunity.” J Virol 93.14 (2019): n. pag.
Influenza virus-infected cells vary widely in their expression of viral genes and only occasionally activate innate immunity. Here, we develop a new method to assess how the genetic variation in viral populations contributes to this heterogeneity. We do this by determining the transcriptome and full-length sequences of all viral genes in single cells infected with a nominally "pure" stock of influenza virus. Most cells are infected by virions with defects, some of which increase the frequency of innate-immune activation. These immunostimulatory defects are diverse and include mutations that perturb the function of the viral polymerase protein PB1, large internal deletions in viral genes, and failure to express the virus's interferon antagonist NS1. However, immune activation remains stochastic in cells infected by virions with these defects and occasionally is triggered even by virions that express unmutated copies of all genes. Our work shows that the diverse spectrum of defects in influenza virus populations contributes to-but does not completely explain-the heterogeneity in viral gene expression and immune activation in single infected cells. Because influenza virus has a high mutation rate, many cells are infected by mutated virions. But so far, it has been impossible to fully characterize the sequence of the virion infecting any given cell, since conventional techniques such as flow cytometry and single-cell transcriptome sequencing (scRNA-seq) only detect if a protein or transcript is present, not its sequence. Here we develop a new approach that uses long-read PacBio sequencing to determine the sequences of virions infecting single cells. We show that viral genetic variation explains some but not all of the cell-to-cell variability in viral gene expression and innate immune induction. Overall, our study provides the first complete picture of how viral mutations affect the course of infection in single cells.
Te Velthuis, Aartjan. “Flu Transcription Captured in Action.” Nat Struct Mol Biol 26.6 (2019): 393–395.
The viral RNA (vRNA) genome of influenza viruses is replicated by the RNA-dependent RNA polymerase (RNAP) via a complementary RNA (cRNA) intermediate. The vRNA promoter can adopt multiple conformations when bound by the RNAP. However, the dynamics, determinants, and biological role of these conformations are unknown; further, little is known about cRNA promoter conformations. To probe the RNA conformations adopted during initial replication, we monitored single, surface-immobilized vRNA and cRNA initiation complexes in real-time. Our results show that, while the 3' terminus of the vRNA promoter exists in dynamic equilibrium between pre-initiation and initiation conformations, the cRNA promoter exhibited very limited dynamics. Two residues in the proximal 3' region of the cRNA promoter (residues absent in the vRNA promoter) allowed the cRNA template strand to reach further into the active site, limiting promoter dynamics. Our results highlight promoter-dependent differences in influenza initiation mechanisms, and advance our understanding of virus replication.

2018

Te Velthuis, Aartjan, Jason Long, and Wendy Barclay. “Assays to Measure the Activity of Influenza Virus Polymerase.” Methods Mol Biol 1836 (2018): 343–374.
Influenza viruses use an RNA-dependent RNA polymerase (RdRp) to transcribe and replicate their segmented negative-stranded RNA genomes. The influenza A virus RdRp consists of a heterotrimeric complex of the proteins PB1, PB2, and PA. The RdRp is associated with the incoming influenza A viral RNA (vRNA) genome bound by the viral nucleoprotein (NP), in complexes called viral ribonucleoproteins, vRNPs. During the viral replication cycle, the RdRp snatches capped primers from nascent host mRNAs to carry out primary viral transcription. Viral mRNA translation produces new copies of the RdRp subunits and NP, which are required to stabilize and encapsidate complementary copies of the genome (cRNAs), forming cRNPs. These cRNPs then use the cRNAs to make new vRNAs, which are encapsidated into new vRNPs. Secondary transcription by new vRNPs results in further viral mRNAs and an increase of the viral protein load in the cell. The activities of the RdRp (mRNA, cRNA, and vRNA synthesis) in the influenza virus replication cycle can be measured on several levels, ranging from assessment of the accumulation of RNA products in virus-infected cells, through in situ reconstitution of the RdRp from cloned cDNAs, to in vitro biochemical assays that allow the dissection of individual functions of the RdRp enzyme. Here we describe these assays and point out the advantages and drawbacks of each.
Goldhill, Daniel et al. “The Mechanism of Resistance to Favipiravir in Influenza.” Proc Natl Acad Sci U S A 115.45 (2018): 11613–11618.
Favipiravir is a broad-spectrum antiviral that has shown promise in treatment of influenza virus infections. While emergence of resistance has been observed for many antiinfluenza drugs, to date, clinical trials and laboratory studies of favipiravir have not yielded resistant viruses. Here we show evolution of resistance to favipiravir in the pandemic H1N1 influenza A virus in a laboratory setting. We found that two mutations were required for robust resistance to favipiravir. We demonstrate that a K229R mutation in motif F of the PB1 subunit of the influenza virus RNA-dependent RNA polymerase (RdRP) confers resistance to favipiravir in vitro and in cell culture. This mutation has a cost to viral fitness, but fitness can be restored by a P653L mutation in the PA subunit of the polymerase. K229R also conferred favipiravir resistance to RNA polymerases of other influenza A virus strains, and its location within a highly conserved structural feature of the RdRP suggests that other RNA viruses might also acquire resistance through mutations in motif F. The mutations identified here could be used to screen influenza virus-infected patients treated with favipiravir for the emergence of resistance.
Te Velthuis, Aartjan et al. “Mini Viral RNAs Act As Innate Immune Agonists During Influenza Virus Infection.” Nat Microbiol 3.11 (2018): 1234–1242.
The molecular processes that determine the outcome of influenza virus infection in humans are multifactorial and involve a complex interplay between host, viral and bacterial factors. However, it is generally accepted that a strong innate immune dysregulation known as 'cytokine storm' contributes to the pathology of infections with the 1918 H1N1 pandemic or the highly pathogenic avian influenza viruses of the H5N1 subtype. The RNA sensor retinoic acid-inducible gene I (RIG-I) plays an important role in sensing viral infection and initiating a signalling cascade that leads to interferon expression. Here, we show that short aberrant RNAs (mini viral RNAs (mvRNAs)), produced by the viral RNA polymerase during the replication of the viral RNA genome, bind to and activate RIG-I and lead to the expression of interferon-β. We find that erroneous polymerase activity, dysregulation of viral RNA replication or the presence of avian-specific amino acids underlie mvRNA generation and cytokine expression in mammalian cells. By deep sequencing RNA samples from the lungs of ferrets infected with influenza viruses, we show that mvRNAs are generated during infection in vivo. We propose that mvRNAs act as the main agonists of RIG-I during influenza virus infection.
Te Velthuis, Aartjan, and Judith Oymans. “Initiation, Elongation, and Realignment During Influenza Virus MRNA Synthesis.” J Virol 92.3 (2018): n. pag.
The RNA-dependent RNA polymerase (RdRp) of the influenza A virus replicates and transcribes the viral genome segments in the nucleus of the host cell. To transcribe these viral genome segments, the RdRp "snatches" capped RNA oligonucleotides from nascent host cell mRNAs and aligns these primers to the ultimate or penultimate nucleotide of the segments for the initiation of viral mRNA synthesis. It has been proposed that this initiation process is not processive and that the RdRp uses a prime-realign mechanism during transcription. Here we provide evidence for the existence of this transcriptional prime-realign mechanism but show that it functions efficiently only for primers that are short or cannot stably base pair with the template. In addition, we demonstrate that transcriptional elongation is dependent on the priming loop of the PB1 subunit of the RdRp. We propose that the prime-realign mechanism may be used to rescue abortive transcription initiation events or cope with sequence variation among primers. Overall, these observations advance our mechanistic understanding of how influenza A virus initiates transcription correctly and efficiently. Influenza A virus causes severe disease in humans and is considered a major global health threat. The virus replicates and transcribes its genome by using an enzyme called the RNA polymerase. To ensure that the genome is amplified faithfully and abundant viral mRNAs are made for viral protein synthesis, the viral RNA polymerase must transcribe the viral genome efficiently. In this report, we characterize a structure inside the polymerase that contributes to the efficiency of viral mRNA synthesis.
Oymans, Judith, and Aartjan Te Velthuis. “A Mechanism for Priming and Realignment During Influenza A Virus Replication.” J Virol 92.3 (2018): n. pag.
The influenza A virus genome consists of eight segments of single-stranded RNA. These segments are replicated and transcribed by a viral RNA-dependent RNA polymerase (RdRp) that is made up of the influenza virus proteins PB1, PB2, and PA. To copy the viral RNA (vRNA) genome segments and the cRNA segments, the replicative intermediate of viral replication, the RdRp must use two promoters and two different initiation mechanisms. On the vRNA promoter, the RdRp initiates on the 3' terminus, while on the cRNA promoter, the RdRp initiates internally and subsequently realigns the nascent vRNA product to ensure that the template is copied in full. In particular, the latter process, which is also used by other RNA viruses, is not understood. Here we provide mechanistic insight into priming and realignment during influenza virus replication and show that it is controlled by the priming loop and a helix-loop-helix motif of the PB1 subunit of the RdRp. Overall, these observations advance our understanding of how the influenza A virus initiates viral replication and amplifies the genome correctly. Influenza A viruses cause severe disease in humans and are considered a major threat to our economy and health. The viruses replicate and transcribe their genome by using an enzyme called the RNA polymerases. To ensure that the genome is amplified faithfully and that abundant viral mRNAs are made for viral protein synthesis, the RNA polymerase must work correctly. In this report, we provide insight into the mechanism that the RNA polymerase employs to ensure that the viral genome is copied correctly.

2017

Nilsson, Benjamin, Aartjan Te Velthuis, and Ervin Fodor. “Role of the PB2 627 Domain in Influenza A Virus Polymerase Function.” J Virol 91.7 (2017): n. pag.
The RNA genome of influenza A viruses is transcribed and replicated by the viral RNA-dependent RNA polymerase, composed of the subunits PA, PB1, and PB2. High-resolution structural data revealed that the polymerase assembles into a central polymerase core and several auxiliary highly flexible, protruding domains. The auxiliary PB2 cap-binding and the PA endonuclease domains are both involved in cap snatching, but the role of the auxiliary PB2 627 domain, implicated in host range restriction of influenza A viruses, is still poorly understood. In this study, we used structure-guided truncations of the PB2 subunit to show that a PB2 subunit lacking the 627 domain accumulates in the cell nucleus and assembles into a heterotrimeric polymerase with PB1 and PA. Furthermore, we showed that a recombinant viral polymerase lacking the PB2 627 domain is able to carry out cap snatching, cap-dependent transcription initiation, and cap-independent ApG dinucleotide extension , indicating that the PB2 627 domain of the influenza virus RNA polymerase is not involved in core catalytic functions of the polymerase. However, in a cellular context, the 627 domain is essential for both transcription and replication. In particular, we showed that the PB2 627 domain is essential for the accumulation of the cRNA replicative intermediate in infected cells. Together, these results further our understanding of the role of the PB2 627 domain in transcription and replication of the influenza virus RNA genome. Influenza A viruses are a major global health threat, not only causing disease in both humans and birds but also placing significant strains on economies worldwide. Avian influenza A virus polymerases typically do not function efficiently in mammalian hosts and require adaptive mutations to restore polymerase activity. These adaptations include mutations in the 627 domain of the PB2 subunit of the viral polymerase, but it still remains to be established how these mutations enable host adaptation on a molecular level. In this report, we characterize the role of the 627 domain in polymerase function and offer insights into the replication mechanism of influenza A viruses.
Posthuma, Clara, Aartjan Te Velthuis, and Eric Snijder. “Nidovirus RNA Polymerases: Complex Enzymes Handling Exceptional RNA Genomes.” Virus Res 234 (2017): 58–73.
Coronaviruses and arteriviruses are distantly related human and animal pathogens that belong to the order Nidovirales. Nidoviruses are characterized by their polycistronic plus-stranded RNA genome, the production of subgenomic mRNAs and the conservation of a specific array of replicase domains, including key RNA-synthesizing enzymes. Coronaviruses (26-34 kilobases) have the largest known RNA genomes and their replication presumably requires a processive RNA-dependent RNA polymerase (RdRp) and enzymatic functions that suppress the consequences of the typically high error rate of viral RdRps. The arteriviruses have significantly smaller genomes and form an intriguing package with the coronaviruses to analyse viral RdRp evolution and function. The RdRp domain of nidoviruses resides in a cleavage product of the replicase polyprotein named non-structural protein (nsp) 12 in coronaviruses and nsp9 in arteriviruses. In all nidoviruses, the C-terminal RdRp domain is linked to a conserved N-terminal domain, which has been coined NiRAN (nidovirus RdRp-associated nucleotidyl transferase). Although no structural information is available, the functional characterization of the nidovirus RdRp and the larger enzyme complex of which it is part, has progressed significantly over the past decade. In coronaviruses several smaller, non-enzymatic nsps were characterized that direct RdRp function, while a 3'-to-5' exoribonuclease activity in nsp14 was implicated in fidelity. In arteriviruses, the nsp1 subunit was found to maintain the balance between genome replication and subgenomic mRNA production. Understanding RdRp behaviour and interactions during RNA synthesis and subsequent processing will be key to rationalising the evolutionary success of nidoviruses and the development of antiviral strategies.

2016

Te Velthuis, Aartjan et al. “The Role of the Priming Loop in RNA Synthesis.” Nat Microbiol 1.5 (2016): n. pag.
RNA-dependent RNA polymerases (RdRps) are used by RNA viruses to replicate and transcribe their RNA genomes1. They adopt a closed, right-handed fold with conserved subdomains called palm, fingers, and thumb1,2. Conserved RdRp motifs A-F coordinate the viral RNA template, NTPs, and magnesium ions to facilitate nucleotide condensation1. For the initiation of RNA synthesis, most RdRps use either a primer-dependent or mechanism3. The RdRp in contrast, uses a capped RNA oligonucleotide to initiate transcription, and a combination of terminal and internal initiation for replication4. To understand how the RdRp coordinates these processes, we analysed the function of a thumb subdomain β-hairpin using initiation, elongation, and single-molecule FRET assays. Our data shows that this β-hairpin is essential for terminal initiation during replication, but auxiliary for internal initiation and transcription. Analysis of individual residues in the tip of the β-hairpin shows that PB1 proline 651 is critical for efficient RNA synthesis and in cell culture. Overall, this work advances our understanding of RNA synthesis and identifies the initiation platform of viral replication.
Te Velthuis, Aartjan, and Ervin Fodor. “Influenza Virus RNA Polymerase: Insights into the Mechanisms of Viral RNA Synthesis.” Nat Rev Microbiol 14.8 (2016): 479–93.
The genomes of influenza viruses consist of multiple segments of single-stranded negative-sense RNA. Each of these segments is bound by the heterotrimeric viral RNA-dependent RNA polymerase and multiple copies of nucleoprotein, which form viral ribonucleoprotein (vRNP) complexes. It is in the context of these vRNPs that the viral RNA polymerase carries out transcription of viral genes and replication of the viral RNA genome. In this Review, we discuss our current knowledge of the structure of the influenza virus RNA polymerase, and insights that have been gained into the molecular mechanisms of viral transcription and replication, and their regulation by viral and host factors. Furthermore, we discuss how advances in our understanding of the structure and function of polymerases could help in identifying new antiviral targets.
Robb, Nicole et al. “Single-Molecule FRET Reveals the Pre-Initiation and Initiation Conformations of Influenza Virus Promoter RNA.” Nucleic Acids Res 44.21 (2016): 10304–10315.
Influenza viruses have a segmented viral RNA (vRNA) genome, which is replicated by the viral RNA-dependent RNA polymerase (RNAP). Replication initiates on the vRNA 3' terminus, producing a complementary RNA (cRNA) intermediate, which serves as a template for the synthesis of new vRNA. RNAP structures show the 3' terminus of the vRNA template in a pre-initiation state, bound on the surface of the RNAP rather than in the active site; no information is available on 3' cRNA binding. Here, we have used single-molecule Förster resonance energy transfer (smFRET) to probe the viral RNA conformations that occur during RNAP binding and initial replication. We show that even in the absence of nucleotides, the RNAP-bound 3' termini of both vRNA and cRNA exist in two conformations, corresponding to the pre-initiation state and an initiation conformation in which the 3' terminus of the viral RNA is in the RNAP active site. Nucleotide addition stabilises the 3' vRNA in the active site and results in unwinding of the duplexed region of the promoter. Our data provide insights into the dynamic motions of RNA that occur during initial influenza replication and has implications for our understanding of the replication mechanisms of similar pathogenic viruses.
Te Velthuis, Aartjan et al. “The Role of the Priming Loop in Influenza A Virus RNA Synthesis.” Nat Microbiol 1 (2016): 16029.
RNA-dependent RNA polymerases (RdRps) are used by RNA viruses to replicate and transcribe their RNA genomes(1). They adopt a closed, right-handed fold with conserved subdomains called palm, fingers and thumb(1,2). Conserved RdRp motifs A-F coordinate the viral RNA template, NTPs and magnesium ions to facilitate nucleotide condensation(1). For the initiation of RNA synthesis, most RdRps use either a primer-dependent or de novo mechanism(3). The influenza A virus RdRp, in contrast, uses a capped RNA oligonucleotide to initiate transcription, and a combination of terminal and internal de novo initiation for replication(4). To understand how the influenza A virus RdRp coordinates these processes, we analysed the function of a thumb subdomain β-hairpin using initiation, elongation and single-molecule Förster resonance energy transfer (sm-FRET) assays. Our data indicate that this β-hairpin is essential for terminal initiation during replication, but not necessary for internal initiation and transcription. Analysis of individual residues in the tip of the β-hairpin shows that PB1 proline 651 is critical for efficient RNA synthesis in vitro and in cell culture. Overall, this work advances our understanding of influenza A virus RNA synthesis and identifies the initiation platform of viral replication.

2014

Most avian influenza viruses do not replicate efficiently in human cells. This is partly due to the low activity of the RNA polymerase of avian influenza viruses in mammalian cells. Nevertheless, this impediment can be overcome through an E→K adaptive mutation at residue 627 of the PB2 subunit of the polymerase. Accordingly, viral ribonucleoprotein (RNP) reconstitution assays show that a viral polymerase containing PB2 627E has impaired activity in mammalian cells compared to a viral polymerase that contains PB2 627K, characteristic of mammalian-adapted influenza viruses. In contrast, purified viral polymerases containing either PB2 627E or PB2 627K show comparable levels of activity in transcription assays that require no RNP assembly. We sought to reconcile these conflicting observations by using an NP-independent cell-based transcription/replication assay to assess viral polymerase activity. We found that PB2 627E polymerase restriction in mammalian cells is independent of NP expression but is dependent on the length of the viral RNA template. In addition, restriction of PB2 627E polymerase was overcome by mutations specific to the viral RNA template promoter sequence. Consequently, we propose that PB2 627E affects recruitment of the viral RNA promoter by the viral polymerase in mammalian cells.
Stubbs, Thomas, and Aartjan Te Velthuis. “The RNA-Dependent RNA Polymerase of the Influenza A Virus.” Future Virol 9.9 (2014): 863–876.
The influenza A virus causes a highly contagious respiratory disease that significantly impacts our economy and health. Its replication and transcription is catalyzed by the viral RNA polymerase. This enzyme is also crucial for the virus, because it is involved in the adaptation of zoonotic strains. It is thus of major interest for the development of antiviral therapies and is being intensively studied. In this article, we will discuss recent advances that have improved our knowledge of the structure of the RNA polymerase and how mutations in the polymerase help the virus to spread effectively among new hosts.
Te Velthuis, Aartjan. “Common and Unique Features of Viral RNA-Dependent Polymerases.” Cell Mol Life Sci 71.22 (2014): 4403–20.
Eukaryotes and bacteria can be infected with a wide variety of RNA viruses. On average, these pathogens share little sequence similarity and use different replication and transcription strategies. Nevertheless, the members of nearly all RNA virus families depend on the activity of a virally encoded RNA-dependent polymerase for the condensation of nucleotide triphosphates. This review provides an overview of our current understanding of the viral RNA-dependent polymerase structure and the biochemistry and biophysics that is involved in replicating and transcribing the genetic material of RNA viruses.

2013

Transcription and replication of the influenza A virus RNA genome are mediated by the viral RNA polymerase from a promoter consisting of the partially base-paired 3' and 5' termini of viral genome segments. Here we show that transcription and replication can be uncoupled by mutation of an unpaired adenosine in the 5' strand of the promoter. This residue is important for transcription but not replication by being essential for the cap-binding activity of the RNA polymerase.

2012

Adedeji, Adeyemi et al. “Mechanism of Nucleic Acid Unwinding by SARS-CoV Helicase.” PLoS One 7.5 (2012): e36521.
The non-structural protein 13 (nsp13) of Severe Acute Respiratory Syndrome Coronavirus (SARS-CoV) is a helicase that separates double-stranded RNA (dsRNA) or DNA (dsDNA) with a 5' → 3' polarity, using the energy of nucleotide hydrolysis. We determined the minimal mechanism of helicase function by nsp13. We showed a clear unwinding lag with increasing length of the double-stranded region of the nucleic acid, suggesting the presence of intermediates in the unwinding process. To elucidate the nature of the intermediates we carried out transient kinetic analysis of the nsp13 helicase activity. We demonstrated that the enzyme unwinds nucleic acid in discrete steps of 9.3 base-pairs (bp) each, with a catalytic rate of 30 steps per second. Therefore the net unwinding rate is ~280 base-pairs per second. We also showed that nsp12, the SARS-CoV RNA-dependent RNA polymerase (RdRp), enhances (2-fold) the catalytic efficiency of nsp13 by increasing the step size of nucleic acid (RNA/RNA or DNA/DNA) unwinding. This effect is specific for SARS-CoV nsp12, as no change in nsp13 activity was observed when foot-and-mouth-disease virus RdRp was used in place of nsp12. Our data provide experimental evidence that nsp13 and nsp12 can function in a concerted manner to improve the efficiency of viral replication and enhance our understanding of nsp13 function during SARS-CoV RNA synthesis.
Te Velthuis, Aartjan, Sjoerd Worm, and Eric Snijder. “The SARS-Coronavirus nsp7+nsp8 Complex Is a Unique Multimeric RNA Polymerase Capable of Both De Novo Initiation and Primer Extension.” Nucleic Acids Res 40.4 (2012): 1737–47.
Uniquely among RNA viruses, replication of the ~30-kb SARS-coronavirus genome is believed to involve two RNA-dependent RNA polymerase (RdRp) activities. The first is primer-dependent and associated with the 106-kDa non-structural protein 12 (nsp12), whereas the second is catalysed by the 22-kDa nsp8. This latter enzyme is capable of de novo initiation and has been proposed to operate as a primase. Interestingly, this protein has only been crystallized together with the 10-kDa nsp7, forming a hexadecameric, dsRNA-encircling ring structure [i.e. nsp(7+8), consisting of 8 copies of both nsps]. To better understand the implications of these structural characteristics for nsp8-driven RNA synthesis, we studied the prerequisites for the formation of the nsp(7+8) complex and its polymerase activity. We found that in particular the exposure of nsp8's natural N-terminal residue was paramount for both the protein's ability to associate with nsp7 and for boosting its RdRp activity. Moreover, this 'improved' recombinant nsp8 was capable of extending primed RNA templates, a property that had gone unnoticed thus far. The latter activity is, however, ~20-fold weaker than that of the primer-dependent nsp12-RdRp at equal monomer concentrations. Finally, site-directed mutagenesis of conserved D/ExD/E motifs was employed to identify residues crucial for nsp(7+8) RdRp activity.

2011

Binding selectivity and cross-reactivity within one of the largest and most abundant interaction domain families, the PDZ family, has long been enigmatic. The complete human PDZ domain complement (the PDZome) consists of 267 domains and we applied here a Bayesian selectivity model to predict hundreds of human PDZ domain interactions, using target sequences of 22,997 non-redundant proteins. Subsequent analysis of these binding scores shows that PDZs can be divided into two genome-wide clusters that coincide well with the division between canonical class 1 and 2 PDZs. Within the class 1 PDZs we observed binding overlap at unprecedented levels, mediated by two residues at positions 1 and 5 of the second α-helix of the binding pocket. Eight PDZ domains were subsequently selected for experimental binding studies and to verify the basics of our predictions. Overall, the PDZ domain class 1 cross-reactivity identified here implies that auxiliary mechanisms must be in place to overcome this inherent functional overlap and to minimize cross-selectivity within the living cell. Indeed, when we superimpose PDZ domain binding affinities with gene ontologies, network topology data and the domain position within a PDZ superfamily protein, functional overlap is minimized and PDZ domains position optimally in the binding space. We therefore propose that PDZ domain selectivity is achieved through cellular context rather than inherent binding specificity.

2010

To accommodate its RNA synthesis in the infected cell, severe acute respiratory syndrome coronavirus (SARS-CoV) induces a cytoplasmic reticulovesicular network (RVN) that is derived from endoplasmic reticulum (ER) membranes. We set out to investigate how the early secretory pathway interacts with the RVN and the viral replication/transcription complex (RTC) that is anchored to it. When the secretory pathway was disrupted by brefeldin A (BFA) treatment at the start of infection, RVN formation and viral RTC activity were not blocked and continued up to 11 h postinfection, although RNA synthesis was reduced by ca. 80%. In vitro RTC assays, using membrane fractions from infected cells, demonstrated that BFA does not directly interfere with the activity of the viral RNA-synthesizing enzymes. Confocal microscopy studies showed that early secretory pathway components are not associated with SARS-CoV-induced replication sites, although our studies revealed that infection induces a remarkable redistribution of the translocon subunit Sec61alpha. Ultrastructural studies, including electron tomography, revealed that the formation of the RVN and all its previously documented features can occur in the presence of BFA, despite differences in the volume and morphology of the network. We therefore conclude that early secretory pathway proteins do not play a direct role in RVN morphogenesis or the functionality of the SARS-CoV RTC. The BFA-induced disruption of ER integrity and functionality probably affects the overall quality of the membrane scaffold that is needed to support the viral RTC and/or the availability of specific host factors, which in turn compromises viral RNA synthesis.
Increasing the intracellular Zn(2+) concentration with zinc-ionophores like pyrithione (PT) can efficiently impair the replication of a variety of RNA viruses, including poliovirus and influenza virus. For some viruses this effect has been attributed to interference with viral polyprotein processing. In this study we demonstrate that the combination of Zn(2+) and PT at low concentrations (2 µM Zn(2+) and 2 µM PT) inhibits the replication of SARS-coronavirus (SARS-CoV) and equine arteritis virus (EAV) in cell culture. The RNA synthesis of these two distantly related nidoviruses is catalyzed by an RNA-dependent RNA polymerase (RdRp), which is the core enzyme of their multiprotein replication and transcription complex (RTC). Using an activity assay for RTCs isolated from cells infected with SARS-CoV or EAV--thus eliminating the need for PT to transport Zn(2+) across the plasma membrane--we show that Zn(2+) efficiently inhibits the RNA-synthesizing activity of the RTCs of both viruses. Enzymatic studies using recombinant RdRps (SARS-CoV nsp12 and EAV nsp9) purified from E. coli subsequently revealed that Zn(2+) directly inhibited the in vitro activity of both nidovirus polymerases. More specifically, Zn(2+) was found to block the initiation step of EAV RNA synthesis, whereas in the case of the SARS-CoV RdRp elongation was inhibited and template binding reduced. By chelating Zn(2+) with MgEDTA, the inhibitory effect of the divalent cation could be reversed, which provides a novel experimental tool for in vitro studies of the molecular details of nidovirus replication and transcription.
Te Velthuis, Aartjan et al. “Quantitative Guidelines for Force Calibration through Spectral Analysis of Magnetic Tweezers Data.” Biophys J 99.4 (2010): 1292–302.
Single-molecule techniques are powerful tools that can be used to study the kinetics and mechanics of a variety of enzymes and their complexes. Force spectroscopy, for example, can be used to control the force applied to a single molecule and thereby facilitate the investigation of real-time nucleic acid-protein interactions. In magnetic tweezers, which offer straightforward control and compatibility with fluorescence measurements or parallel tracking modes, force-measurement typically relies on the analysis of positional fluctuations through video microscopy. Significant errors in force estimates, however, may arise from incorrect spectral analysis of the Brownian motion in the magnetic tweezers. Here we investigated physical and analytical optimization procedures that can be used to improve the range over which forces can be reliably measured. To systematically probe the limitations of magnetic tweezers spectral analysis, we have developed a magnetic tweezers simulator, whose outcome was validated with experimental data. Using this simulator, we evaluate methods to correctly perform force experiments and provide guidelines for correct force calibration under configurations that can be encountered in typical magnetic tweezers experiments.
Te Velthuis, Aartjan et al. “The RNA Polymerase Activity of SARS-Coronavirus nsp12 Is Primer Dependent.” Nucleic Acids Res 38.1 (2010): 203–14.
An RNA-dependent RNA polymerase (RdRp) is the central catalytic subunit of the RNA-synthesizing machinery of all positive-strand RNA viruses. Usually, RdRp domains are readily identifiable by comparative sequence analysis, but biochemical confirmation and characterization can be hampered by intrinsic protein properties and technical complications. It is presumed that replication and transcription of the approximately 30-kb severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) RNA genome are catalyzed by an RdRp domain in the C-terminal part of nonstructural protein 12 (nsp12), one of 16 replicase subunits. However, thus far full-length nsp12 has proven refractory to expression in bacterial systems, which has hindered both the biochemical characterization of coronavirus RNA synthesis and RdRp-targeted antiviral drug design. Here, we describe a combined strategy involving bacterial expression of an nsp12 fusion protein and its in vivo cleavage to generate and purify stable SARS-CoV nsp12 (106 kDa) with a natural N-terminus and C-terminal hexahistidine tag. This recombinant protein possesses robust in vitro RdRp activity, as well as a significant DNA-dependent activity that may facilitate future inhibitor studies. The SARS-CoV nsp12 is primer dependent on both homo- and heteropolymeric templates, supporting the likeliness of a close enzymatic collaboration with the intriguing RNA primase activity that was recently proposed for coronavirus nsp8.

2008

Ott, Elisabeth et al. “The Lim Domain Only Protein 7 Is Important in Zebrafish Heart Development.” Dev Dyn 237.12 (2008): 3940–52.
The LIM domain only protein 7 (LMO7), a member of the PDZ and LIM domain-containing protein family is a candidate gene with possible roles in embryonic development and breast cancer progression. LMO7 has been linked to actin cytoskeleton organization through nectin/afadin and to cell-cell adhesion by means of E-cadherin/catenin. In addition, LMO7 has been shown to regulate transcription of the nuclear membrane protein Emerin and other muscle relevant genes. In this study, we used in situ hybridization to investigate LMO7 expression during embryonic development in three widely used vertebrate model species: the zebrafish, the chicken and the mouse. Our temporal and spatial gene expression analysis revealed both common and distinct patterns between these species. In mouse and chicken embryos we found expression in the outflow tract, the inflow tract, the pro-epicardial organ and the second heart field, structures highly important in the developing heart. Furthermore, gene knockdown experiments in zebrafish embryos resulted in severe defects in heart development with effects on the conduction system and on heart localization. In summary, we present here the first developmental study of LMO7. We reveal the temporal and spatial expression patterns of this important gene during mouse, chicken and fish development and our findings suggest essential functions for LMO7 during vertebrate heart development.

2007

Te Velthuis, Aartjan, Jeroen Admiraal, and Christoph Bagowski. “Molecular Evolution of the MAGUK Family in Metazoan Genomes.” BMC Evol Biol 7 (2007): 129.
BACKGROUND: Development, differentiation and physiology of metazoans all depend on cell to cell communication and subsequent intracellular signal transduction. Often, these processes are orchestrated via sites of specialized cell-cell contact and involve receptors, adhesion molecules and scaffolding proteins. Several of these scaffolding proteins important for synaptic and cellular junctions belong to the large family of membrane-associated guanylate kinases (MAGUK). In order to elucidate the origin and the evolutionary history of the MAGUKs we investigated full-length cDNA, EST and genomic sequences of species in major phyla. RESULTS: Our results indicate that at least four of the seven MAGUK subfamilies were present in early metazoan lineages, such as Porifera. We employed domain sequence and structure based methods to infer a model for the evolutionary history of the MAGUKs. Notably, the phylogenetic trees for the guanylate kinase (GK)-, the PDZ- and the SH3-domains all suggested a matching evolutionary model which was further supported by molecular modeling of the 3D structures of different GK domains. We found no MAGUK in plants, fungi or other unicellular organisms, which suggests that the MAGUK core structure originated early in metazoan history. CONCLUSION: In summary, we have characterized here the molecular and structural evolution of the large MAGUK family. Using the MAGUKs as an example, our results show that it is possible to derive a highly supported evolutionary model for important multidomain families by analyzing encoded protein domains. It further suggests that larger superfamilies encoded in the different genomes can be analyzed in a similar manner.
The PDZ and LIM domain-containing protein family is encoded by a diverse group of genes whose phylogeny has currently not been analyzed. In mammals, ten genes are found that encode both a PDZ- and one or several LIM-domains. These genes are: ALP, RIL, Elfin (CLP36), Mystique, Enigma (LMP-1), Enigma homologue (ENH), ZASP (Cypher, Oracle), LMO7 and the two LIM domain kinases (LIMK1 and LIMK2). As conventional alignment and phylogenetic procedures of full-length sequences fell short of elucidating the evolutionary history of these genes, we started to analyze the PDZ and LIM domain sequences themselves. Using information from most sequenced eukaryotic lineages, our phylogenetic analysis is based on full-length cDNA-, EST-derived- and genomic- PDZ and LIM domain sequences of over 25 species, ranging from yeast to humans. Plant and protozoan homologs were not found. Our phylogenetic analysis identifies a number of domain duplication and rearrangement events, and shows a single convergent event during evolution of the PDZ/LIM family. Further, we describe the separation of the ALP and Enigma subfamilies in lower vertebrates and identify a novel consensus motif, which we call 'ALP-like motif' (AM). This motif is highly-conserved between ALP subfamily proteins of diverse organisms. We used here a combinatorial approach to define the relation of the PDZ and LIM domain encoding genes and to reconstruct their phylogeny. This analysis allowed us to classify the PDZ/LIM family and to suggest a meaningful model for the molecular evolution of the diverse gene architectures found in this multi-domain family.
Ott, Elisabeth, Aartjan Te Velthuis, and Christoph Bagowski. “Comparative Analysis of Splice Form-Specific Expression of LIM Kinases During Zebrafish Development.” Gene Expr Patterns 7.5 (2007): 620–9.
LIM Kinases (LIMK) are genes encoding multi-domain proteins that can contain up to two LIM domains, a single PDZ domain, and a tyrosine kinase domain. Alternative splicing is a source for different combinations of these domains. Two family members, LIMK1 and LIMK2 have been described in mammals and are important for organization of the actin cytoskeleton. We have cloned LIMK1 and LIMK2 from zebrafish and characterized their domain specific expression patterns during embryogenesis. The results on temporal and spatial expression of the LIM Kinases during embryogenesis indicate overlapping and distinct expression domains for LMK1 and LIMK2. Differences in expression during embryogenesis were observed for PDZ and LIM encoding splice forms for both LIM Kinases. To better understand the transcriptional regulation of LIM Kinases, we searched for conserved regulatory elements. We identified evolutionary conserved smad binding sites for LIMK2. In summary, we present here the splice-form specific temporal and spatial expression patterns for both LIMK1 and LIMK2 during zebrafish embryogenesis.
Te Velthuis, Aartjan et al. “Gene Expression Patterns of the ALP Family During Zebrafish Development.” Gene Expr Patterns 7.3 (2007): 297–305.
The actinin-associated LIM protein (ALP) genes belong to the PDZ/LIM protein family which is characterized by the presence of both a PDZ and a LIM domain. The ALP subfamily in mammals has four members: ALP, Elfin, Mystique and RIL. In this study, we have annotated and cloned the zebrafish ALP gene family and identified a zebrafish-specific fifth member of the family, the alp-like gene. We compared the zebrafish sequences to their human and mouse orthologues. A phylogenetic analysis based on the amino acid sequences showed the overall high degree of conservation within the family. We describe here the expression patterns for all five ALP family genes during zebrafish development. Whole mount in situ hybridization results revealed common and distinct expression patterns for the five genes. With the exception of elfin, all genes were expressed as maternal RNAs at early developmental stages. Gene expression for all of them appeared regulated and localized in specific regions at the eight different developmental stages studied. Expression for all five genes was observed in the central nervous system (CNS), which led us to further investigate brain-specific expression in sections of embryos at 2 days of development. In summary, we identified the zebrafish orthologues of the ALP family and determined their gene expression patterns during zebrafish embryogenesis. Finally, we compare our results to the limited expression data available for this gene family during mammalian development.