Biology subjects

Ott, M.

Publications and source records attributed to Ott, M..

At least 19 recordsLinked to original sources

bioRxiv2026

Genome-wide within-host evolution during prolonged SARS-CoV-2 infection in a person with advanced HIV infection

Within-host evolution during prolonged SARS-CoV-2 infection can be extensive in immunocompromised individuals, but the consequences of mutations that emerge and change in frequency over time remain difficult to predict. We characterized an XBB.1.5.10 infection lasting nearly 90 days in a person with advanced human immunodeficiency virus (HIV) infection. Genome-wide tracking of allele frequencies in samples collected on study days 43, 48, and 70 revealed a Spike N-terminal domain (NTD) deletion spanning residues 138-145 at frequencies of 33%, 80%, and 96%, respectively. A receptor-binding domain (RBD) deletion-insertion spanning residues 373-378 was detected on days 43 and 48 but not on day 70, while nsp12 C464Y was detected at approximately 8% on day 48 and 86% on day 70. We reconstructed representative Spike and non-Spike mutation profiles from the longitudinal samples and tested their effects on cell entry and viral RNA replication. The representative Spike construct for days 43 and 48 increased entry relative to XBB.1.5. Comparison with a matched construct lacking the receptor-binding domain deletion-insertion showed that the deletion-insertion enhanced entry, although it was not detected in the day 70 sample. The day 48 non-Spike profile supported the highest RNA replication among the tested replicons. Together, allele-frequency tracking revealed both transient mutations and mutations detected at progressively higher frequencies across the sampled time points. Functional testing showed that increased entry or RNA replication in vitro did not necessarily predict persistence in vivo.

microbiology↗

bioRxiv2026

Bead-based RNA Detection with Cas13a: Multi-scale Design and Validation

CRISPR-based assays offer sensitive and specific nucleic acid detection for a broad range of diagnostic and surveillance applications. While standard solution-based assays are simple, they typically lack the ability to detect multiple targets individually without the need for more complex instrumentation and multi-step workflows. Surface-based strategies offer greater potential for multiplexing since the identity of a target can be associated with a location on a slide or a specific bead, as in the case of DNA arrays or Luminex bead assays. However, the design principles for optimizing surface-based CRISPR assays have yet to be developed. Here we present a multi-scale analytical framework integrating nanoscale enzyme kinetics, microscale diffusion constraints, and macroscale assay parameters that we use to develop a bead-based Cas13a assay where the bead becomes fluorescent when the target RNA for that bead is detected. The assay, which we call SurfCas, uses surface-bound guide RNAs and reporter RNAs to detect one or more soluble target RNAs in a one-pot assay. We show that SurfCas has a sensitivity approaching bulk reactions, and we demonstrate multiplexed detection of target RNAs in a complex biological matrix. The design principles we establish for surface-based CRISPR assays offer guidance on how to further improve SurfCas sensitivity and scale multiplexed detection of target RNAs in a single reaction.

bioengineering↗

bioRxiv2026

Epistasis between SARS-CoV-2 M and N Proteins Balances Particle Assembly and Immune Evasion

Since its emergence in the human population, SARS-CoV-2 has continuously evolved to evade immune responses and robustly establish global circulation. In this process, the structural viral membrane (M) protein has accumulated amino acid changes whose impact on viral particle assembly and innate immune evasion remains incompletely understood. Here, we designed a SARS-CoV-2 replicon system lacking M that assesses the influence of transiently transfected M protein variants on viral particle production independently of viral RNA replication. We found that M protein variants have reduced particle assembly while innate immune antagonism functions are strengthened. Notably, the assembly defect is rescued by co-evolving N protein variants, highlighting how SARS-CoV-2 evolution coordinates between two of its structural proteins to optimize viral infection. Our work underscores the complex evolutionary trajectories of SARS-CoV-2 variants across different viral proteins and informs future therapeutic strategies targeting viral assembly and limiting infection. Author summarySince the onset of the COVID-19, SARS-CoV-2 has been changing continuously in ways that help it spread efficiently and evade human defenses. The viral membrane (M) protein is a key structural component required to form new viral particles. Nevertheless, it has accumulated changes over time with overlooked functions. In this study, we developed an experimental system to examine the impact of these changes on virus assembly and host immune system. We found that recent M variants are better than older variants at suppressing immune responses but less efficient at forming viral particles. This defect seems to be compensated for by coordinated changes in another structural protein, nucleocapsid (N). These findings reveal that SARS-CoV-2 evolution involves trade-offs between different viral characteristics, with changes in one protein balancing changes in another. Understanding these trade-offs provides new insight into how the virus adapts to humans and may help guide therapeutic strategies to limit infection.

microbiology↗

bioRxiv2026

Pandemic Coronavirus Genome Packaging Relies on Multiple Dispersed Packaging Signals

Packaging of viral genomes into progeny virions is a critical step in the viral life cycle. Coronaviruses such as SARS-CoV-2 possess unusually large RNA genomes ([~]30 kb), yet the mechanism by which these genomes are selectively condensed and incorporated into virions remains poorly understood. Here, we demonstrate that the SARS-CoV-2 genome contains multiple dispersed RNA structures, termed packaging signals (PSs), which cooperate with a dominant central PS to direct genomic RNA incorporation into infectious particles. We identify the dominant PS within the coding region of the nsp15 gene, downstream of a packaging signal previously described in Embecoviruses. Using virus-like particles (VLPs), we investigate its role in virion assembly and selective genomic RNA packaging, and show that it promotes the formation of ribonucleoprotein (RNP) complexes with the viral nucleocapsid protein (N), as revealed by mass photometry. Notably, we uncover a unique N-induced conformational rearrangement of the PS RNA, from an extended structure to a double stem-loop architecture. This dominant PS acts together with a nearby stem-loop element to assemble a higher-order RNP complex containing 12 N-protein dimers. Using a SARS-CoV-2 replicon system, we further demonstrate functional cooperativity between the dominant PS, its proximal partner stem-loop, and additional packaging elements located approximately 10 kb upstream. Collectively, our findings support a highly dynamic and cooperative mechanism of SARS-CoV-2 genome packaging that relies on multiple dispersed packaging signals organized around a dominant central PS. These insights provide a mechanistic framework for understanding coronavirus genome packaging and reveal new opportunities for antiviral intervention through disruption of this process. They also have important implications for vaccine development and may enable the design of membrane-based vector systems capable of efficiently delivering large nucleic acid cargoes, expanding the potential of bionanotechnology and genetic medicine.

microbiology↗

bioRxiv2026

Secreted ORF8 reprograms macrophages to enhance SARS-CoV-2 infection of lung epithelial cells

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) primarily targets the respiratory epithelium, yet severe disease features diffuse lung injury and hyperinflammatory syndromes driven by dysregulated immune activation. Emerging evidence indicates that resident and infiltrating immune cells in the lung can encounter the virus early in infection and, under specific conditions, become infected (1, 2). This process amplifies local inflammation and facilitates viral propagation in the lower airways and distal lung regions (3, 4), where angiotensin converting enzyme 2 (ACE2) expression is limited (5, 6). However, how epithelial and immune cell compartments interact to produce the hallmark pulmonary pathology of SARS-CoV-2 infection remains unresolved. Here we show that secreted ORF8, a SARS-CoV-2 accessory protein, drives inflammatory lung pathology by increasing macrophage permissiveness to infection, triggering pyroptosis, and amplifying viral replication in alveolar epithelial cells. Co-culture of macrophages with human alveolar type II (AT2) cells overrides ORF8s previously reported inhibition of AT2 infection (7, 8), restoring productive viral replication. In vivo, IL-17RA blockade counteracts ORF8 activity, lowering viral burden and attenuating pulmonary inflammation and fibrosis. These findings reveal a paracrine role for ORF8 in reprogramming macrophages, thereby establishing a feedforward proviral circuit that accelerates lung pathology in COVID-19 and are clinically relevant given the recurrent emergence of SARS-CoV-2 variants with either intact or deleted ORF8 since the beginning of the pandemic.

microbiology↗

bioRxiv2026

Characterization of genetically effective cells and EMS mutagenesis on the novel winter oil seed Pennycress (Thlaspi arvense)

Pennycress (Thlaspi arvense L.) is an intermediate winter oilseed crop that has only recently been domesticated for agronomic use. Improving agronomic traits requires sources of genetic variation, and mutagenesis is frequently used to help overcome the limitations of natural populations. We investigate the impact of Ethyl methanesulfonate (EMS) on genetically effective cells (GECs) to characterize the intra-individual genetic variation of EMS mutagenesis in pennycress. We identified that pennycress contains at least 4 GECs which, when treated with EMS, create unique mutations across different branches within the same individual plant. We then propagated the M2 plants for whole genome sequencing, providing extensive characterization of the EMS mutation profile and developing a gene index as a resource for future reverse genetic screenings. Article SummaryPennycress is an emerging winter oil seed crop in the American Midwest. Domestication efforts have advanced rapidly through a combination of genetic techniques. One of the most successful methods has been the use of a mutant gene index, a large collection of pennycress seed where new genetic variation has been created through Ethyl methanesulfonate (EMS). EMS mutations are not uniform however, and a single treated seed can have wide genetic variation within the resulting plant. We investigate the role of genetically effective cells on EMS variation, and present the full EMS population as a resource for further pennycress domestication efforts.

genomics↗

bioRxiv2026

LUCas: Light-Uncaged Cas13a using photocleavable interfering guide RNAs

CRISPR diagnostics have emerged as powerful tools for detecting infectious diseases, with the RNA endonuclease Cas13a enabling sensitive and specific, amplification-free RNA detection through collateral trans-cleavage of fluorescent reporters. However, background cleavage from unbound enzyme, contaminating nucleases, and unsynchronized initiation of reactions limits assay sensitivity and interpretability. A strategy to precisely control the onset of Cas13a catalytic activity, essentially a molecular "starting gun", would address these challenges and expand assay design space. Here, we introduce Light-Uncaged Cas13a (LUCas), a light controllable system that directly gates Cas13a using a photocleavable interfering guide RNA (pc-igRNA) that suppresses trans-cleavage activity even in the presence of target RNA. Brief UV illumination releases this suppression, restoring full activity. Quantitative kinetic analysis reveals an approximately 100-fold suppression of trans-cleavage activity prior to photo-activation. Importantly, LUCas also suppresses target-independent background activity, enabling a predictive, background-limited determination of assay sensitivity. Using measured kinetic parameters, we predict and experimentally validate the limit-of-detection of the LUCas system. Finally, we demonstrate a multiplexed detection strategy termed "temporal barcoding," which enables quantitative detection of viral co-infections in a single bulk reaction. Together, these results establish LUCas as a general framework for mechanistically informed, light-based control of Cas13a activity.

bioengineering↗

bioRxiv2026

Coupling high-throughput protease enzymology with viral replication reveals biochemical constraints of viral fitness

Proteases govern essential biological processes and are key drug targets, yet how protease sequence variation quantitatively reshapes biochemical parameters and constrains biological fitness remains poorly understood. Here, we integrate high-throughput in vitro enzymology with cellular assays to link protease sequence, biochemistry, and fitness. We extend a microfluidic platform for high-throughput protease enzymology (HT-MEKpro), which is broadly applicable across protease families and catalytic classes, enabling measurement of catalytic turnover (kcat), Michaelis constant (KM), inhibitor potency (IC50), and relative substrate specificity for 102-103 variants. Applied to the SARS-CoV-2 main protease (Mpro), HT-MEKpro generated parallel catalytic and inhibitory landscapes for >400 variants. Integration with viral replication and in-cell cleavage assays reveals that variants with altered substrate specificity fail to support replication, suggesting imbalanced polyprotein processing as a constraint on viral fitness. More broadly, these data can enable mechanistically grounded modeling of protease sequence-property relationships and inform strategies for pharmacological modulation beyond active-site inhibition.

biochemistry↗

bioRxiv2025

Drug-induced phospholipidosis as an artifact in antiviral drug repurposing

Drug repurposing in principle can speed antiviral drug discovery. Among the molecules most frequently advanced in such repurposing efforts are a group of structurally diverse cationic amphiphilic drugs (CADs). While CADs have shown micromolar to mid-nanomolar antiviral activity in cell-based assays, they can induce phospholipidosis, confounding in COVID-19 repurposing. A barrier to the identification of phospholipidosis inducers has been the involved nature of the microscopy assays used to characterize them. To ease the identification of these artifacts, we describe a rapid microplate-based assay to detect phospholipidosis. Leveraging this assay, we quantified the prevalence of phospholipidosis-inducers across several cell-based antiviral repurposing screens. We selected 40 drugs reported to have micromolar antiviral activities and found that 26 of them (65%) induced phospholipidosis within the same concentration range as their reported antiviral activities. Intriguingly, we identified four non-CADs that also induce phospholipidosis, revealing a new group of drugs that can lead to this toxic event and highlighting the importance of facile experimental assays to detect it. Understanding how phospholipidosis can confound antiviral drug discovery, and its rapid detection, will help prevent what is an apparently general artifact, active across viruses, from distracting investigators from potentially more useful candidates.

cell biology↗

bioRxiv2025

Discovery of AVI-6451, a Potent and Selective Inhibitor of the SARS-CoV-2 ADP-Ribosylhydrolase Mac1 with Oral Efficacy in vivo

The COVID-19 pandemic made plain the need for effective antivirals acting on novel antiviral targets, among which viral macrodomains have attracted considerable attention. We recently described AVI-4206 (1), a potent and selective inhibitor of the SARS-CoV-2 ADP-ribosylhydrolase Mac1 based on a 9H-pyrimido[4,5-b]indole core, the first Mac1 inhibitor to demonstrate antiviral efficacy in mouse models of SARS-CoV-2 infection, but requiring IP administration and frequent dosing. Herein we describe an extensive, structurally enabled medicinal chemistry effort to identify orally bioavailable Mac1 inhibitors by addressing permeability and efflux liabilities of 1 and many of its analogs. Multiple strategies were pursued to overcome these issues, including replacing a urea function to reduce hydrogen bond donor count. While heterocyclic urea mimetics could deliver analogs like AVI-6318 (3) with potencies and ADME profiles similar to 1, abrogation of the P-gp liability was finally achieved with entirely non-polar substituents in place of urea. Thus, AVI-6451 (4) is a potent Mac1 inhibitor lead with low intrinsic clearance, high oral bioavailability, and antiviral efficacy with once-daily oral administration in a mouse model of SARS-CoV-2 infection.

pharmacology and toxicology↗

bioRxiv2025

Hypoimmunogenic human motor neurons induced from iPSCs in vivo substantially ameliorate ALS disease in large animal models

Stem cell-based therapy holds great potential for substituting degenerated motor neurons (MNs) in amyotrophic lateral sclerosis (ALS). Missing protocols for advanced differentiation of transplanted cells into MNs, immune rejection, and the lack of suitable ALS models for preclinical trials have slowed the development of effective therapies. Here, we employed multiplex genetic-editing to generate a novel human pluripotent stem cell line containing doxycycline (Dox)-inducible MNs-specific transcription factors and comprehensively modified immunomodulatory genes. We transplanted these cells into the spinal cord of ALS large animal models (SOD1G93A pigs and TIA1P362L rabbits), which faithfully recapitulate pathologies and symptoms observed in ALS patients. The transplanted cells could efficiently differentiate into functional MNs upon Dox treatment in vivo, distribute throughout the spinal cord and motor cortex via extensive migration, survive long-term without the need for immunosuppression. Notably, these MNs integrated into host neural circuits, as evidenced by their long projection of peripheral axons to target muscle and reformation of neuromuscular junctions. As result, pathologies and motor deficits were substantially ameliorated in both animal models. One Sentence SummaryHypoimmunogenic human motor neurons induced from iPSCs in vivo reform neuromuscular junctions and ameliorate ALS disease in pig and rabbit models.

neuroscience↗

bioRxiv2025

Programmable translational inhibition by a molecular glue-oligonucleotide conjugate

Selective inhibition of mRNA translation is a promising strategy for modulating the activity of disease-associated genes, yet achieving both high potency and specificity remains challenging. Rocaglamide A (RocA), a molecular glue, inhibits translation by clamping eIF4A onto polypurine motifs found in many transcripts, thereby limiting RocAs specificity. Here, we developed RocASO, a chemical conjugate that links RocA to an antisense oligonucleotide (ASO) capable of base-pairing with defined mRNA sequences, thus directing RocAs clamping mechanism to chosen targets and enhancing overall potency and specificity. We show that RocASOs are compatible with various types of ASO modalities, including gapmers that induce the degradation of target RNAs. RocASOs were designed to effectively knock down endogenous genes (PTGES3, HSPA1B) and SARS-CoV-2 viral RNA, the latter conferring potent antiviral activity in cells. These findings establish RocASO as a versatile platform for programmable translational inhibition with therapeutic potential.

genomics↗

bioRxiv2025

Structural Acrobatics of IAA7: Exercising Fuzzy Regions to Aid Auxin-Mediated Catalysis

Auxin is the paragon of small molecules driving recognition of proteins for their ubiquitylation and degradation, enabling rapid decoding of external signals into transcriptional responses. Auxin functions as a molecular glue, enhancing interactions between the TRANSPORT INHIBITOR RESPONSE 1 (TIR1) and other TIR1-like proteins with various AUXIN INDUCIBLE/INDOLE-3-ACETIC ACID (AUX/IAA) transcriptional repressors. A Cullin RING E3 ubiquitin ligase, SCFTIR1/AFBs, mediates the ubiquitylation and degradation of AUX/IAA proteins by positioning them for ubiquitin transfer, leading to their rapid degradation by the 26S proteasome. AUX/IAAs contain a Phox Bem 1 (PB1) domain, a degron, and intrinsically disordered regions (IDRs) that influence auxin sensing and their polyubiquitylation by interacting with TIR1. However, the precise role of IDRs in AUX/IAA positioning for ubiquitylation, and the mechanism of multisite ubiquitylation within AUX/IAAs remain unclear. We used biophysical techniques and coarse-grained simulations (CGS) to study the structural behavior of IDRs in IAA7, a model AUX/IAA protein from Arabidopsis thaliana. Our findings show that IAA7 expands by approximately 2.6 nm upon binding to TIR1 in the presence of auxin, likely increasing the proximity between a lysine acceptor in IAA7 and the active site of the SCFTIR1 E3 ligase for ubiquitylation. Our data also suggest that IAA7 remains conformationally heterogeneous when bound to TIR1, forming numerous transient contacts along its IDRs and C-terminal PB1 with TIR1. We propose that these transient interactions outside the IAA7 degron interface enhance the TIR1{middle dot}IAA7 association, resulting in highly dynamic and heterogeneous complexes that facilitate IAA7 multisite ubiquitylation, targeting it for rapid proteasomal degradation. Significance StatementIn this study, we use biophysical techniques to elucidate the implications of AUX/IAA intrinsic disorder on their conformational ensemble and processing by the E3 ubiquitin ligase, SCFTIR1. Our findings demonstrate that IAA7 undergoes a structural expansion upon binding to TIR1 and remains highly dynamic, facilitating multisite ubiquitylation. These insights enhance our understanding of how ubiquitylation targets engage with their E3 receptor modules and underscore the structural advantages of intrinsic disorder in protein ubiquitylation targets.

biophysics↗

bioRxiv2025

A non-spike nucleocapsid R204P mutation in SARS-CoV-2 Omicron XEC enhances inflammation and pathogenicity

The global circulation of SARS-CoV-2 in human populations has driven the emergence of Omicron subvariants, which have become highly diversified through recombination. In late 2024, SARS-CoV-2 Omicron XEC variant emerged from the recombination of two JN.1 progeny, KS.1.1 and KP.3.3, and became predominant worldwide. Here, we investigated virological features of the XEC variant. Epidemic dynamics modeling suggested that spike substitutions in XEC mainly contribute to its increased viral fitness. Additionally, four licensed antivirals were effective against XEC. Although the fusogenicity of XEC spike is comparable to that of the JN.1 spike, the intrinsic pathogenicity of XEC in hamsters was significantly higher than that of JN.1. Notably, we found that the nucleocapsid R204P mutation of XEC enhanced inflammation through NF-{kappa}B activation. Recent studies suggest that the evolutionary potential of spike protein is reaching its limit. Indeed, our findings highlight the critical role of non-spike mutations in the future evolution of SARS-CoV-2.

microbiology↗

bioRxiv2025

A molecular switch at the yeast mitoribosomal tunnel exit controls cytochrome b synthesis

Mitochondrial gene expression needs to be balanced with cytosolic translation to produce oxidative phosphorylation complexes. In yeast, translational feedback loops involving lowly expressed proteins called translational activators help to achieve this balance. Synthesis of cytochrome b (Cytb or COB), a core subunit of complex III in the respiratory chain, is controlled by three translational activators and the assembly factor Cbp3-Cbp6. However, the molecular interface between the COB translational feedback loop and complex III assembly is yet unknown. Here, using protein-proximity mapping combined with selective mitoribosome profiling, we reveal the components and dynamics of the molecular switch controlling COB translation. Specifically, we demonstrate that Mrx4, a previously uncharacterized ligand of the mitoribosomal polypeptide tunnel exit, interacts with either the assembly factor Cbp3-Cbp6 or with the translational activator Cbs2. These reciprocal interactions determine whether the translational activator complex with bound COB mRNA can interact with the mRNA channel exit on the small ribosomal subunit for translation initiation. Organization of the feedback loop at the tunnel exit therefore orchestrates mitochondrial translation with respiratory chain biogenesis.

molecular biology↗

bioRxiv2025

Translational activators align mRNAs at the small mitoribosomal subunit for translation initiation

Mitochondrial gene expression is essential for oxidative phosphorylation. Mitochondrial-encoded mRNAs are translated by dedicated mitochondrial ribosomes (mitoribosomes), whose regulation remains elusive. In the bakers yeast Saccharomyces cerevisiae, nuclear-encoded mitochondrial translational activators (TAs) facilitate transcript-specific translation by a yet unknown mechanism. Here, we investigated the function of TAs containing RNA-binding pentatricopeptide repeats (PPRs) using selective mitoribosome profiling and cryo-EM structural analysis. These analyses revealed that TAs exhibit strong selectivity for mitoribosomes initiating on their target transcripts. Moreover, TA-mitoribosome footprints indicated that TAs recruit mitoribosomes proximal to the start codon. Two cryo-EM structures of mRNA-TA complexes bound to post-initiation/pre-elongation-stalled mitoribosomes revealed the general mechanism of TA action. Specifically, the TAs bind to structural elements in the 5 UTR of the client mRNA as well as to the mRNA channel exit to align the mRNA in the small subunit during initiation. Our findings provide a mechanistic basis for understanding how mitochondria achieve transcript-specific translation initiation without relying on general sequence elements to position mitoribosomes at start codons.

molecular biology↗

bioRxiv2025

Structure-based discovery of highly bioavailable, covalent, broad-spectrum coronavirus-MPro inhibitors with potent in vivo efficacy

The main protease (MPro) of SARS-CoV-2 is crucial for viral replication and is the target of nirmatrelvir (the active ingredient of Paxlovid) and ensitrelvir. The identification of new agents with differentiated pharmacokinetic and drug resistance profiles will increase therapeutic options for COVID-19 patients and bolster pandemic preparedness generally. Starting with a lead-like dihydrouracil chemotype from a large-library docking campaign, we improved MPro inhibition >1,000-fold by engaging additional sub-sites in the MPro active site, most notably by employing a latent propargyl electrophile to engage the catalytic Cys145. Advanced leads from this series, including AVI-4516 and AVI-4773 show pan-coronavirus antiviral activity in cells, very low clearance in mice, and for AVI-4773 a rapid reduction in viral titers more than a million-fold after just three doses, more rapidly and effectively than the approved drugs, nirmatrelvir and ensitrelvir. Both AVI-4516 and AVI-4773 are well distributed in mouse tissues, including brain, where concentrations ten or fifteen-thousand times the EC90, respectively, are observed eight hours after an oral dose. As exemplar of the series, AVI-4516 shows minimal inhibition of major CYP isoforms and human cysteine and serine proteases, likely due to its latent-electrophilic warhead. AVI-4516 also exhibits synergy in cellular infection models in combination with the RdRp inhibitor molnupiravir, while related analogs strongly inhibit nirmatrelvir-resistant MPro mutant virus in cells. The in vivo and antiviral properties of this new chemotype are differentiated from existing clinical and pre-clinical MPro inhibitors, and will advance new therapeutic development against emerging SARS-CoV-2 variants and other coronaviruses. One sentence summaryThis manuscript describes the discovery of a new class of potent inhibitors of the SARS-CoV-2 major proteases (MPro) with a unique mechanism of inhibition, pan coronaviral activity in cellulo, exquisite selectivity vs. the human proteome, and exceptional in vivo efficacy in SARS-CoV-2 infection models that surpasses that of currently approved agents.

biochemistry↗

bioRxiv2024

Enhanced RNA replication and pathogenesis in recent SARS-CoV-2 variants harboring the L260F mutation in NSP6

The COVID-19 pandemic has been driven by SARS-CoV-2 variants with enhanced transmission and immune escape. Apart from extensive evolution in the Spike protein, non-Spike mutations are accumulating across the entire viral genome and their functional impact is not well understood. To address the contribution of these mutations, we reconstructed genomes of recent Omicron variants with disabled Spike expression (replicons) to systematically compare their RNA replication capabilities independently from Spike. We also used a single reference replicon and complemented it with various Omicron variant Spike proteins to quantify viral entry capabilities in single-round infection assays. Viral entry and RNA replication were negatively correlated, suggesting that as variants evolve reduced entry functions under growing immune pressure on Spike, RNA replication increases as a compensatory mechanism. We identified multiple mutations across the viral genome that enhanced viral RNA replication. NSP6 emerged as a hotspot with a distinct L260F mutation independently arising in the BQ.1.1 and XBB.1.16 variants. Using mutant and revertant NSP6 viral clones, the L260F mutation was validated to enhance viral replication in cells and increase pathogenesis in mice. Notably, this mutation enhanced host lipid droplet consumption by NSP6 without impacting its known ER-zippering function or double-membrane vesicle morphology. Collectively, a systematic analysis of RNA replication of recent Omicron variants defined NSP6s key role in viral RNA replication that provides insight into evolutionary trajectories of recent variants with possible therapeutic implications. Author SummaryAs SARS-CoV-2 continues to spread and adapt in humans, viral variants with enhanced spread and immune evasion have emerged throughout the COVID-19 pandemic. While most of the mutations occur in the Spike protein and have been extensively studied, non-Spike mutations have been accumulating and are not as well understood. Here, we constructed Spike-defective genomes of recent Omicron variants and systematically compared their RNA replication capabilities independently from Spike. We also performed single-round infection assays with various Omicron variant Spike proteins to quantify viral entry capabilities. Interestingly, viral entry and RNA replication were negatively correlated, suggesting that as variants evolved reduced entry functions under growing immune pressure on Spike, RNA replication increased as a compensatory mechanism. We focused on a viral protein NSP6 that acquired mutations that significantly enhanced RNA replication. We validated that a frequently accessed L260F mutation in NSP6 enhanced viral infection in cells and increased pathogenesis in mice. While the mutation did not alter NSP6s role in maintaining a membranous network of viral replication factories, the mutation enhanced viral hijacking of the host lipid droplet machinery. Collectively, we highlight the important role of non-Spike mutations in the evolutionary trajectories of SARS-CoV-2 variants with possible monitoring and therapeutic implications.

microbiology↗