Ultrastructure of influenza virus ribonucleoprotein complexes during viral RNA synthesis

Nano-Scale Video Imaging of Motility Machinery by High-Speed Atomic Force Microscopy

Motility is a vital aspect of many forms of life, with a wide range of highly conserved as well as highly unique systems adapted to the needs of various organisms and environments. While many motility systems are well studied using structural techniques like X-ray crystallography and electron microscopy, as well as fluorescence microscopy methodologies, it is difficult to directly determine the relationship between the shape and movement of a motility system due to a notable gap in spatiotemporal resolution. Bridging this gap as well as understanding the dynamic molecular movements that underpin motility mechanisms has been challenging. The advent of high-speed atomic force microscopy (HS-AFM) has provided a new window into understanding these nano-scale machines and the dynamic processes underlying motility. In this review, we highlight some of the advances in this field, ranging from reconstituted systems and purified higher-order supramolecular complexes to live cells, in both prokaryotic and eukaryotic contexts.

Structure of the tilapia lake virus nucleoprotein bound to RNA

Tilapia Lake virus (TiLV) belongs to the Amnoonviridae family within the Articulavirales order of segmented negative-strand RNA viruses and is highly diverged from more familiar orthomyxoviruses, such as influenza. The viral nucleoprotein (NP), a key component of the replication machinery, packages the viral genome into protective ribonucleoprotein particles. Here we describe the electron cryo-microscopy (cryo-EM) structure of TiLV-NP bound to RNA within in vitro reconstituted, small ring-like, pseudo-symmetrical oligomers. Although TiLV-NP is considerably smaller than its influenza counterpart and unrelated in sequence, it maintains the same topology and domain organisation. This comprises a head and body domain between which is a positively charged groove, where single-stranded RNA binds. In addition, an oligomerisation loop inserts into a hydrophobic pocket in the neighbouring NP, the flexible hinges of which allow variable orientation of adjacent NPs. Focused cryo-EM maps unambiguously define the 5' to 3' direction of the bound RNA, confirmed by double stranded, A-form RNA regions that extrude out from some of the NP-NP interfaces. This is the first fully resolved description of how single-stranded and stem-loop RNA binds to an articulaviral NP assembly. Superposition with orthomyxoviral NPs suggest that the mode of RNA binding is likely similar across the Articulavirales order.

Sample Preparation for Cryo-Electron Tomography of Influenza A Virus and Infected Cells

Influenza A virus (IAV) replication within host cells relies on tightly controlled, dynamic interactions between viral and cellular components. Yet we have a limited understanding of the viral replication cycle at molecular resolution. Electron microscopy of resin-embedded samples has traditionally been employed to investigate viral replication in cells. However, this technique relies on chemical fixation of samples, dehydration, and resin embedding, which leads to loss of molecular details. Cryo-electron tomography emerged as an indispensable tool enabling the exploration of viral replication within vitrified samples, providing an unperturbed environment. Here, we provide a sample preparation protocol for in vitro and cellular cryo-electron tomography of isolated IAV, as well as for cryo-focused ion beam milling of IAV-infected cells at different stages of infection. The emphasis of this chapter is on infection conditions, grid handling, and adaptation of microscopy settings to the individual sample while adhering to biosafety regulations.

Purification and Ultramicroscopic Observation of the Influenza A Virus Ribonucleoprotein Complex

Influenza A virus (IAV) has an eight-segmented, single-stranded, negative-sense viral genomic RNA (vRNA). Each vRNA strand associates with nucleoproteins and an RNA-dependent RNA polymerase complex to form a viral ribonucleoprotein (vRNP) complex. IAV vRNPs adopt a flexible double-helical configuration that varies in length. Although the transcription and replication of vRNA take place in the context of vRNPs, the precise structural conformation of vRNPs during RNA synthesis remains partially elucidated. To unravel the intricate ultrastructure of the vRNP, it is necessary to purify it while preserving its native functionality. Herein, we introduce a comprehensive protocol for the purification of IAV vRNPs using glycerol gradient ultracentrifugation. Furthermore, we provide a method for the high-speed atomic force microscopy observation of vRNPs during viral RNA synthesis.

In vitro one-pot construction of influenza viral genomes for virus particle synthesis based on reverse genetics system

The reverse genetics system, which allows the generation of influenza viruses from plasmids encoding viral genome, is a powerful tool for basic research on viral infection mechanisms and application research such as vaccine development. However, conventional plasmid construction using Escherichia coli (E.coli) cloning is time-consuming and has difficulties handling DNA encoding genes toxic for E.coli or highly repeated sequences. These limitations hamper rapid virus synthesis. In this study, we establish a very rapid in vitro one-pot plasmid construction (IVOC) based virus synthesis. This method dramatically reduced the time for genome plasmid construction, which was used for virus synthesis, from several days or more to about 8 hours. Moreover, infectious viruses could be synthesized with a similar yield to the conventional E.coli cloning-based method with high accuracy. The applicability of this method was also demonstrated by the generation of recombinant viruses carrying reporter genes from the IVOC products. This method enables the pathogenicity analysis and vaccine development using genetically modified viruses, and it is expected to allow for faster analysis of newly emerging variants than ever before. Furthermore, its application to other RNA viruses is also expected.

Structural characterization of Thogoto Virus nucleoprotein provides insights into viral RNA encapsidation and RNP assembly

Orthomyxoviruses, such as influenza and thogotoviruses, are important human and animal pathogens. Their segmented viral RNA genomes are wrapped by viral nucleoproteins (NPs) into helical ribonucleoprotein complexes (RNPs). NP structures of several influenza viruses have been reported. However, there are still contradictory models of how orthomyxovirus RNPs are assembled. Here, we characterize the crystal structure of Thogoto virus (THOV) NP and found striking similarities to structures of influenza viral NPs, including a two-lobed domain architecture, a positively charged RNA-binding cleft, and a tail loop important for trimerization and viral transcription. A low-resolution cryo-electron tomography reconstruction of THOV RNPs elucidates a left-handed double helical assembly. By providing a model for RNP assembly of THOV, our study suggests conserved NP assembly and RNA encapsidation modes for thogoto- and influenza viruses.

Mapping of the influenza A virus genome RNA structure and interactions reveals essential elements of viral replication

Influenza A virus (IAV) represents a constant public health threat. The single-stranded, segmented RNA genome of IAV is replicated in host cell nuclei as a series of 8 ribonucleoprotein complexes (vRNPs) with RNA structures known to exert essential function to support viral replication. Here, we investigate RNA secondary structures and RNA interactions networks of the IAV genome and construct an in vivo structure model for each of the 8 IAV genome segments. Our analyses reveal an overall in vivo and in virio resemblance of the IAV genome conformation but also wide disparities among long-range and intersegment interactions. Moreover, we identify a long-range RNA interaction that exerts an essential role in genome packaging. Disrupting this structure displays reduced infectivity, attenuating virus pathogenicity in mice. Our findings characterize the in vivo RNA structural landscape of the IAV genome and reveal viral RNA structures that can be targeted to develop antiviral interventions.

Structural Impact of the Interaction of the Influenza A Virus Nucleoprotein with Genomic RNA Segments

Influenza A viruses (IAVs) possess a segmented genome consisting of eight viral RNAs (vRNAs) associated with multiple copies of viral nucleoprotein (NP) and a viral polymerase complex. Despite the crucial role of RNA structure in IAV replication, the impact of NP binding on vRNA structure is not well understood. In this study, we employed SHAPE chemical probing to compare the structure of NS and M vRNAs of WSN IAV in various states: before the addition of NP, in complex with NP, and after the removal of NP. Comparison of the RNA structures before the addition of NP and after its removal reveals that NP, while introducing limited changes, remodels local structures in both vRNAs and long-range interactions in the NS vRNA, suggesting a potentially biologically relevant RNA chaperone activity. In contrast, NP significantly alters the structure of vRNAs in vRNA/NP complexes, though incorporating experimental data into RNA secondary structure prediction proved challenging. Finally, our results suggest that NP not only binds single-stranded RNA but also helices with interruptions, such as bulges or small internal loops, with a preference for G-poor and C/U-rich regions.

Cryo-EM structure of influenza helical nucleocapsid reveals NP-NP and NP-RNA interactions as a model for the genome encapsidation

Influenza virus genome encapsidation is essential for the formation of a helical viral ribonucleoprotein (vRNP) complex composed of nucleoproteins (NP), the trimeric polymerase, and the viral genome. Although low-resolution vRNP structures are available, it remains unclear how the viral RNA is encapsidated and how NPs assemble into the helical filament specific of influenza vRNPs. In this study, we established a biological tool, the RNP-like particles assembled from recombinant influenza A virus NP and synthetic RNA, and we present the first subnanometric cryo-electron microscopy structure of the helical NP-RNA complex (8.7 to 5.3 Å). The helical RNP-like structure reveals a parallel double-stranded conformation, allowing the visualization of NP-NP and NP-RNA interactions. The RNA, located at the interface of neighboring NP protomers, interacts with conserved residues previously described as essential for the NP-RNA interaction. The NP undergoes conformational changes to enable RNA binding and helix formation. Together, our findings provide relevant insights for understanding the mechanism for influenza genome encapsidation.

Challenges in Imaging Analyses of Biomolecular Condensates in Cells Infected with Influenza A Virus

Biomolecular condensates are crucial compartments within cells, relying on their material properties for function. They form and persist through weak, transient interactions, often undetectable by classical biochemical approaches. Hence, microscopy-based techniques have been the most reliable methods to detail the molecular mechanisms controlling their formation, material properties, and alterations, including dissolution or phase transitions due to cellular manipulation and disease, and to search for novel therapeutic strategies targeting biomolecular condensates. However, technical challenges in microscopy-based analysis persist. This paper discusses imaging, data acquisition, and analytical methodologies' advantages, challenges, and limitations in determining biophysical parameters explaining biomolecular condensate formation, dissolution, and phase transitions. In addition, we mention how machine learning is increasingly important for efficient image analysis, teaching programs what a condensate should resemble, aiding in the correlation and interpretation of information from diverse data sources. Influenza A virus forms liquid viral inclusions in the infected cell cytosol that serve as model biomolecular condensates for this study. Our previous work showcased the possibility of hardening these liquid inclusions, potentially leading to novel antiviral strategies. This was established using a framework involving live cell imaging to measure dynamics, internal rearrangement capacity, coalescence, and relaxation time. Additionally, we integrated thermodynamic characteristics by analysing fixed images through Z-projections. The aforementioned paper laid the foundation for this subsequent technical paper, which explores how different modalities in data acquisition and processing impact the robustness of results to detect bona fide phase transitions by measuring thermodynamic traits in fixed cells. Using solely this approach would greatly simplify screening pipelines. For this, we tested how single focal plane images, Z-projections, or volumetric analyses of images stained with antibodies or live tagged proteins altered the quantification of thermodynamic measurements. Customizing methodologies for different biomolecular condensates through advanced bioimaging significantly contributes to biological research and potential therapeutic advancements.

In vivo secondary structural analysis of Influenza A virus genomic RNA

Influenza A virus (IAV) is a respiratory virus that causes epidemics and pandemics. Knowledge of IAV RNA secondary structure in vivo is crucial for a better understanding of virus biology. Moreover, it is a fundament for the development of new RNA-targeting antivirals. Chemical RNA mapping using selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) coupled with Mutational Profiling (MaP) allows for the thorough examination of secondary structures in low-abundance RNAs in their biological context. So far, the method has been used for analyzing the RNA secondary structures of several viruses including SARS-CoV-2 in virio and in cellulo. Here, we used SHAPE-MaP and dimethyl sulfate mutational profiling with sequencing (DMS-MaPseq) for genome-wide secondary structure analysis of viral RNA (vRNA) of the pandemic influenza A/California/04/2009 (H1N1) strain in both in virio and in cellulo environments. Experimental data allowed the prediction of the secondary structures of all eight vRNA segments in virio and, for the first time, the structures of vRNA5, 7, and 8 in cellulo. We conducted a comprehensive structural analysis of the proposed vRNA structures to reveal the motifs predicted with the highest accuracy. We also performed a base-pairs conservation analysis of the predicted vRNA structures and revealed many highly conserved vRNA motifs among the IAVs. The structural motifs presented herein are potential candidates for new IAV antiviral strategies.

Polymorphic self-assembly of helical tubules is kinetically controlled

In contrast to most self-assembling synthetic materials, which undergo unbounded growth, many biological self-assembly processes are self-limited. That is, the assembled structures have one or more finite dimensions that are much larger than the size scale of the individual monomers. In many such cases, the finite dimension is selected by a preferred curvature of the monomers, which leads to self-closure of the assembly. In this article, we study an example class of self-closing assemblies: cylindrical tubules that assemble from triangular monomers. By combining kinetic Monte Carlo simulations, free energy calculations, and simple theoretical models, we show that a range of programmable size scales can be targeted by controlling the intricate balance between the preferred curvature of the monomers and their interaction strengths. However, their assembly is kinetically controlled-the tubule morphology is essentially fixed shortly after closure, resulting in a distribution of tubule widths that is significantly broader than the equilibrium distribution. We develop a simple kinetic model based on this observation and the underlying free-energy landscape of assembling tubules that quantitatively describes the distributions. Our results are consistent with recent experimental observations of tubule assembly from triangular DNA origami monomers. The modeling framework elucidates design principles for assembling self-limited structures from synthetic components, such as artificial microtubules that have a desired width and chirality.

Tools shaping drug discovery and development

Spectroscopic, scattering, and imaging methods play an important role in advancing the study of pharmaceutical and biopharmaceutical therapies. The tools more familiar to scientists within industry and beyond, such as nuclear magnetic resonance and fluorescence spectroscopy, serve two functions: as simple high-throughput techniques for identification and purity analysis, and as potential tools for measuring dynamics and structures of complex biological systems, from proteins and nucleic acids to membranes and nanoparticle delivery systems. With the expansion of commercial small-angle x-ray scattering instruments into the laboratory setting and the accessibility of industrial researchers to small-angle neutron scattering facilities, scattering methods are now used more frequently in the industrial research setting, and probe-less time-resolved small-angle scattering experiments are now able to be conducted to truly probe the mechanism of reactions and the location of individual components in complex model or biological systems. The availability of atomic force microscopes in the past several decades enables measurements that are, in some ways, complementary to the spectroscopic techniques, and wholly orthogonal in others, such as those related to nanomechanics. As therapies have advanced from small molecules to protein biologics and now messenger RNA vaccines, the depth of biophysical knowledge must continue to serve in drug discovery and development to ensure quality of the drug, and the characterization toolbox must be opened up to adapt traditional spectroscopic methods and adopt new techniques for unraveling the complexities of the new modalities. The overview of the biophysical methods in this review is meant to showcase the uses of multiple techniques for different modalities and present recent applications for tackling particularly challenging situations in drug development that can be solved with the aid of fluorescence spectroscopy, nuclear magnetic resonance spectroscopy, atomic force microscopy, and small-angle scattering.

Secondary Structure of Influenza A Virus Genomic Segment 8 RNA Folded in a Cellular Environment

Influenza A virus (IAV) is a member of the single-stranded RNA (ssRNA) family of viruses. The most recent global pandemic caused by the SARS-CoV-2 virus has shown the major threat that RNA viruses can pose to humanity. In comparison, influenza has an even higher pandemic potential as a result of its high rate of mutations within its relatively short (<13 kbp) genome, as well as its capability to undergo genetic reassortment. In light of this threat, and the fact that RNA structure is connected to a broad range of known biological functions, deeper investigation of viral RNA (vRNA) structures is of high interest. Here, for the first time, we propose a secondary structure for segment 8 vRNA (vRNA8) of A/California/04/2009 (H1N1) formed in the presence of cellular and viral components. This structure shows similarities with prior in vitro experiments. Additionally, we determined the location of several well-defined, conserved structural motifs of vRNA8 within IAV strains with possible functionality. These RNA motifs appear to fold independently of regional nucleoprotein (NP)-binding affinity, but a low or uneven distribution of NP in each motif region is noted. This research also highlights several accessible sites for oligonucleotide tools and small molecules in vRNA8 in a cellular environment that might be a target for influenza A virus inhibition on the RNA level.

Migration of Influenza Virus Nucleoprotein into the Nucleolus Is Essential for Ribonucleoprotein Complex Formation

Influenza A virus double-helical ribonucleoprotein complex (RNP) performs transcription and replication of viral genomic RNA (vRNA). Although RNP formation occurs in the nuclei of virus-infected cells, the nuclear domains involved in this process remain unclear. Here, we show that the nucleolus is an essential site for functional RNP formation. Viral nucleoprotein (NP), a major RNP component, temporarily localized to the nucleoli of virus-infected cells. Mutations in a nucleolar localization signal (NoLS) on NP abolished double-helical RNP formation, resulting in a loss of viral RNA synthesis ability, whereas ectopic fusion of the NoLS enabled the NP mutant to form functional double-helical RNPs. Furthermore, nucleolar disruption of virus-infected cells inhibited NP assembly into double-helical RNPs, resulting in decreased viral RNA synthesis. Collectively, our findings demonstrate that NP migration into the nucleolus is a critical step for functional RNP formation, showing the importance of the nucleolus in the influenza virus life cycle.

Visualising Viruses

Viruses pose a challenge to our imaginations. They exert a highly visible influence on the world in which we live, but operate at scales we cannot directly perceive and without a clear separation between their own biology and that of their hosts. Communication about viruses is therefore typically grounded in mental images of virus particles. Virus particles, as the infectious stage of the viral replication cycle, can be used to explain many directly observable properties of transmission, infection and immunity. In addition, their often striking beauty can stimulate further interest in virology. The structures of some virus particles have been determined experimentally in great detail, but for many important viruses a detailed description of the virus particle is lacking. This can be because they are challenging to describe with a single experimental method, or simply because of a lack of data. In these cases, methods from medical illustration can be applied to produce detailed visualisations of virus particles which integrate information from multiple sources. Here, we demonstrate how this approach was used to visualise the highly variable virus particles of influenza A viruses and, in the early months of the COVID-19 pandemic, the virus particles of the then newly characterised and poorly described SARS-CoV-2. We show how constructing integrative illustrations of virus particles can challenge our thinking about the biology of viruses, as well as providing tools for science communication, and we provide a set of science communication resources to help visualise two viruses whose effects are extremely apparent to all of us.