Regulation of virion production by the ORF8 signal peptide across SARS-CoV-2 variants (preprint)

The open reading frame 8 (ORF8), an accessory protein of SARS-CoV-2, is prone to deletions and mutations across different viral variants, which was first described in several Singapore variants. The reason why viral evolution favors loss or inactivation of ORF8 is not fully understood, although the effects of ORF8 on inflammation, immune evasion, and disease severity have been described. Here we show using clinical ORF8 deficient viral isolates, virus like particles (VLPs) and viral replicons that ORF8 expression dampens viral particle production. ORF8 physically interacts with the viral Spike protein and induces Golgi fragmentation, overall contributing to less virus particle production. Using systematic ORF8 deletions, we mapped the particle reducing function to its N terminal signal peptide. Interestingly, this part of ORF8 is severely truncated in the recent XBB.1.5 variant, and when restored, suppresses viral particle production in the context of the entire viral genome. Collectively, our data support the model that evolutionary pressure exists to delete ORF8 sequence and expression across SARS-CoV-2 variants to fully enable viral particle production.

Assembly reactions of SARS-CoV-2 nucleocapsid protein with nucleic acid (preprint)

The viral genome of SARS-CoV-2 is packaged by the nucleocapsid (N-) protein into ribonucleoprotein particles (RNPs), 38{+/-}10 of which are contained in each virion. Their architecture has remained unclear due to the pleomorphism of RNPs, the high flexibility of N-protein intrinsically disordered regions, and highly multivalent interactions between viral RNA and N-protein binding sites in both N-terminal (NTD) and C-terminal domain (CTD). Here we explore critical interaction motifs of RNPs by applying a combination of biophysical techniques to mutant proteins binding different nucleic acids in an in vitro assay for RNP formation, and by examining mutant proteins in a viral assembly assay. We find that nucleic acid-bound N-protein dimers oligomerize via a recently described protein-protein interface presented by a transient helix in its long disordered linker region between NTD and CTD. The resulting hexameric complexes are stabilized by multi-valent protein-nucleic acid interactions that establish crosslinks between dimeric subunits. Assemblies are stabilized by the dimeric CTD of N-protein offering more than one binding site for stem-loop RNA. Our study suggests a model for RNP assembly where N- protein scaffolding at high density on viral RNA is followed by cooperative multimerization through protein-protein interactions in the disordered linker.

Assembly of SARS-CoV-2 ribonucleosomes by truncated N* variant of the nucleocapsid protein (preprint)

The Nucleocapsid (N) protein of SARS-CoV-2 compacts the RNA genome into viral ribonucleoprotein (vRNP) complexes within virions. Assembly of vRNPs is inhibited by phosphorylation of the N protein SR region. Several SARS-CoV-2 variants of concern carry N protein mutations that reduce phosphorylation and enhance the efficiency of viral packaging. Variants of the dominant B.1.1 viral lineage also encode a truncated N protein, termed N* or {Delta}(1-209), that mediates genome packaging despite lacking the N-terminal RNA-binding domain and SR region. Here, we show that {Delta}(1-209) and viral RNA assemble into vRNPs that are remarkably similar in size and shape to those formed with full-length N protein. We show that assembly of {Delta}(1-209) vRNPs requires the leucine-rich helix (LH) of the central disordered region, and that the LH promotes N protein oligomerization. We also find that fusion of a phosphomimetic SR region to {Delta}(1-209) inhibits RNA binding and vRNP assembly. Our results provide new insights into the mechanisms by which RNA binding promotes N protein self-association and vRNP assembly, and how this process is modulated by SR phosphorylation.

Rapid assembly of SARS-CoV-2 genomes reveals attenuation of the Omicron BA.1 variant through NSP6 (preprint)

Although the SARS-CoV-2 Omicron variant (BA.1) spread rapidly across the world and effectively evaded immune responses, its viral fitness in cell and animal models was reduced. The precise nature of this attenuation remains unknown as generating replication-competent viral genomes is challenging because of the length of the viral genome (30kb). Here, we designed a plasmid-based viral genome assembly and rescue strategy (pGLUE) that constructs complete infectious viruses or noninfectious subgenomic replicons in a single ligation reaction with >80% efficiency. Fully sequenced replicons and infectious viral stocks can be generated in 1 and 3 weeks, respectively. By testing a series of naturally occurring viruses as well as Delta-Omicron chimeric replicons, we show that Omicron nonstructural protein 6 harbors critical attenuating mutations, which dampen viral RNA replication and reduce lipid droplet consumption. Thus, pGLUE overcomes remaining barriers to broadly study SARS-CoV-2 replication and reveals deficits in nonstructural protein function underlying Omicron attenuation.

Global landscape of the host response to SARS-CoV-2 variants reveals viral evolutionary trajectories (preprint)

A series of SARS-CoV-2 variants of concern (VOCs) have evolved in humans during the COVID-19 pandemic: Alpha, Beta, Gamma, Delta, and Omicron. Here, we used global proteomic and genomic analyses during infection to understand the molecular responses driving VOC evolution. We discovered VOC-specific differences in viral RNA and protein expression levels, including for N, Orf6, and Orf9b, and pinpointed several viral mutations responsible. An analysis of the host response to VOC infection and comprehensive interrogation of altered virus-host protein-protein interactions revealed conserved and divergent regulation of biological pathways. For example, regulation of host translation was highly conserved, consistent with suppression of VOC replication in mice using the translation inhibitor plitidepsin. Conversely, modulation of the host inflammatory response was most divergent, where we found Alpha and Beta, but not Omicron BA.1, antagonized interferon stimulated genes (ISGs), a phenotype that correlated with differing levels of Orf6. Additionally, Delta more strongly upregulated proinflammatory genes compared to other VOCs. Systematic comparison of Omicron subvariants revealed BA.5 to have evolved enhanced ISG and proinflammatory gene suppression that similarly correlated with Orf6 expression, effects not seen in BA.4 due to a mutation that disrupts the Orf6-nuclear pore interaction. Our findings describe how VOCs have evolved to fine-tune viral protein expression and protein-protein interactions to evade both innate and adaptive immune responses, offering a likely explanation for increased transmission in humans.

Neutralizing immunity in vaccine breakthrough infections from the SARS-CoV-2 Omicron and Delta variants (preprint)

Virus-like particle (VLP) and live virus assays were used to investigate neutralizing immunity to Delta and Omicron SARS-CoV-2 variants in 239 samples from 125 fully vaccinated individuals. In uninfected, non-boosted individuals, VLP neutralization titers to Delta and Omicron were reduced 2.7-fold and 15.4-fold, respectively, compared to wild-type (WT), while boosted individuals (n=23) had 18-fold increased titers. Delta breakthrough infections (n=39) had 57-fold and 3.1-fold titers whereas Omicron breakthrough infections (n=14) had 5.8-fold and 0.32-fold titers compared to uninfected non-boosted and boosted individuals, respectively. The difference in titers (p=0.049) was related to a higher proportion of moderate to severe infections in the Delta cohort (p=0.014). Correlation of neutralizing and spike quantitative antibody titers was decreased with Delta or Omicron compared to WT. Neutralizing antibodies in Delta and Omicron breakthrough infections increase overall, but the relative magnitude of increase is greater in more clinically severe infection and against the specific infecting variant.

Limited Cross-Variant Immunity after Infection with the SARS-CoV-2 Omicron Variant Without Vaccination (preprint)

SARS-CoV-2 Delta and Omicron strains are the most globally relevant variants of concern (VOCs). While individuals infected with Delta are at risk to develop severe lung disease 1 , Omicron infection causes less severe disease, mostly upper respiratory symptoms 2,3 . The question arises whether rampant spread of Omicron could lead to mass immunization, accelerating the end of the pandemic. Here we show that infection with Delta, but not Omicron, induces broad immunity in mice. While sera from Omicron-infected mice only neutralize Omicron, sera from Delta-infected mice are broadly effective against Delta and other VOCs, including Omicron. This is not observed with the WA1 ancestral strain, although both WA1 and Delta elicited a highly pro-inflammatory cytokine response and replicated to similar titers in the respiratory tracts and lungs of infected mice as well as in human airway organoids. Pulmonary viral replication, pro-inflammatory cytokine expression, and overall disease progression are markedly reduced with Omicron infection. Analysis of human sera from Omicron and Delta breakthrough cases reveals effective cross-variant neutralization induced by both viruses in vaccinated individuals. Together, our results indicate that Omicron infection enhances preexisting immunity elicited by vaccines, but on its own may not induce broad, cross-neutralizing humoral immunity in unvaccinated individuals.

Omicron mutations enhance infectivity and reduce antibody neutralization of SARS-CoV-2 virus-like particles (preprint)

The Omicron SARS-CoV-2 virus contains extensive sequence changes relative to the earlier arising B.1, B.1.1 and Delta SARS-CoV-2 variants that have unknown effects on viral infectivity and response to existing vaccines. Using SARS-CoV-2 virus-like particles (SC2-VLPs), we examined mutations in all four structural proteins and found that Omicron showed increased infectivity relative to B.1, B.1.1 and similar to Delta, a property conferred by S and N protein mutations. Thirty-eight antisera samples from individuals vaccinated with tozinameran (Pfizer/BioNTech), elasomeran (Moderna), Johnson & Johnson vaccines and convalescent sera from unvaccinated COVID-19 survivors had moderately to dramatically reduced efficacy to prevent cell transduction by VLPs containing the Omicron mutations. The Pfizer/BioNTech and Moderna vaccine antisera showed strong neutralizing activity against VLPs possessing the ancestral spike protein (B.1, B.1.1), with 3-fold reduced efficacy against Delta and 15-fold lower neutralization against Omicron VLPs. Johnson & Johnson antisera showed minimal neutralization of any of the VLPs tested. Furthermore, the monoclonal antibody therapeutics Casirivimab and Imdevimab had robust neutralization activity against B.1, B.1.1 or Delta VLPs but no detectable neutralization of Omicron VLPs. Our results suggest that Omicron is at least as efficient at assembly and cell entry as Delta, and the antibody response triggered by existing vaccines or previous infection, at least prior to boost, will have limited ability to neutralize Omicron. In addition, some currently available monoclonal antibodies will not be useful in treating Omicron-infected patients.

Launching a saliva-based SARS-CoV-2 surveillance testing program on a university campus (preprint)

Regular surveillance testing of asymptomatic individuals for SARS-CoV-2 has played a vital role in SARS-CoV-2 outbreak prevention on college and university campuses. Here we describe the voluntary saliva testing program instituted at the University of California, Berkeley during an early period of the SARS-CoV-2 pandemic in 2020. The program was administered as a research study ahead of clinical implementation, enabling us to launch surveillance testing while continuing to optimize the assay. Results of both the testing protocol itself and the study participants experience show how the program succeeded in providing routine, robust testing capable of contributing to outbreak prevention within a campus community and offer strategies for encouraging participation and a sense of civic responsibility.

Robotic RNA extraction for SARS-CoV-2 surveillance using saliva samples (preprint)

Saliva is an attractive specimen type for asymptomatic surveillance of COVID-19 in large populations due to its ease of collection and its demonstrated utility for detecting RNA from SARS-CoV-2. Multiple saliva-based viral detection protocols use a direct-to-RT-qPCR approach that eliminates nucleic acid extraction but can reduce viral RNA detection sensitivity. To improve test sensitivity while maintaining speed, we developed a robotic nucleic acid extraction method for detecting SARS-CoV-2 RNA in saliva samples with high throughput. Using this assay, the Free Asymptomatic Saliva Testing (IGI-FAST) research study on the UC Berkeley campus conducted 11,971 tests on supervised self-collected saliva samples and identified rare positive specimens containing SARS-CoV-2 RNA during a time of low infection prevalence. In an attempt to increase testing capacity, we further adapted our robotic extraction assay to process pooled saliva samples. We also benchmarked our assay against the gold standard, nasopharyngeal swab specimens. Finally, we designed and validated a RT-qPCR test suitable for saliva self-collection. These results establish a robotic extraction-based procedure for rapid PCR-based saliva testing that is suitable for samples from both symptomatic and asymptomatic individuals.

Duplex formation between the template and the nascent strand in the transcription-regulating sequences determines the site of template switching in SARS - CoV-2 (preprint)

Recently published transcriptomic data of the SARS-CoV-2 coronavirus show that there is a large variation in the frequency and steady state levels of subgenomic mRNA sequences. This variation is derived from discontinuous subgenomic RNA synthesis where the polymerase switches template from a 3 proximal genome body sequence to a 5 untranslated leader sequence. This leads to a fusion between the common 5 leader sequence and a 3 proximal body sequence in the RNA product. This process revolves around a common core sequence (CS) that is present at both the template sites that make up the fusion junction. Base-pairing between the leader CS and the nascent complementary minus strand body CS, and flanking regions (together called the transcription regulating sequence, TRS) is vital for this template switching event. However, various factors can influence the site of template switching within the same TRS duplex. Here, we model the duplexes formed between the leader and complementary body TRS regions, hypothesising the role of the stability of the TRS duplex in determining the major sites of template switching for the most abundant mRNAs. We indicate that the stability of secondary structures and the speed of transcription play key roles in determining the probability of template switching in the production of subgenomic RNAs.

EVs analysis in the COVID-19 era: insights on serum inactivation protocols towards downstream isolation and analysis (preprint)

Since the outbreak of COVID-19 crisis, the handling of biological samples from confirmed or suspected SARS-CoV-2 positive individuals demanded the use of inactivation protocols to ensure laboratory operators safety. While not standardized, these practices can be roughly divided in two categories, namely heat inactivation and solvent-detergent treatments. As such, these routine procedures should also apply to samples intended for Extracellular Vesicles (EVs) analysis. Assessing the impact of virus inactivating pre-treatments is therefore of pivotal importance, given the well-known variability introduced by different pre-analytical steps on downstream EVs isolation and analysis. Arguably, shared guidelines on inactivation protocols tailored to best address EVs-specific requirements will be needed among the EVs community, yet deep investigations in this direction havent been reported so far. In the attempt of sparking interest on this highly relevant topic, we here provide preliminary insights on SARS-CoV-2 inactivation practices to be adopted prior serum EVs analysis by comparing solvent/detergent treatment vs. heat inactivation. Our analysis entailed the evaluation of EVs recovery and purity along with biochemical, biophysical and biomolecular profiling by means of Nanoparticle Tracking Analysis, Western Blotting, Atomic Force Microscopy, miRNA content (digital droplet PCR) and tetraspanin assessment by microarrays. Our data suggest an increase in ultracentrifugation (UC) recovery following heat-treatment, however accompanied by a marked enrichment in EVs-associated contaminants. On the contrary, solvent/detergent treatment is promising for small EVs (< 150 nm range), yet a depletion of larger vesicular entities was detected. This work represents a first step towards the identification of optimal serum inactivation protocols targeted to EVs analysis.

IGI-LuNER: single-well multiplexed RT-qPCR test for SARS-CoV-2 (preprint)

Commonly used RT-qPCR-based SARS-CoV-2 diagnostics require 2-3 separate reactions or rely on detection of a single viral target, adding time and cost or risk of false-negative results. Currently, no test combines detection of widely used SARS-CoV-2 E- and N-gene targets and a sample control in a single, multiplexed reaction. We developed the IGI-LuNER RT-qPCR assay using the Luna Probe Universal One-Step RT-qPCR master mix with publicly available primers and probes to detect SARS-CoV-2 N gene, E gene, and human RNase P (NER). This combined, cost-effective test can be performed in 384-well plates with detection sensitivity suitable for clinical reporting, and will aid in future sample pooling efforts, thus improving throughput of SARS-CoV-2 detection. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=79 SRC="FIGDIR/small/20247338v2_ufig1.gif" ALT="Figure 1"> View larger version (27K): org.highwire.dtl.DTLVardef@74929corg.highwire.dtl.DTLVardef@1457971org.highwire.dtl.DTLVardef@2825ddorg.highwire.dtl.DTLVardef@1cde2b6_HPS_FORMAT_FIGEXP M_FIG C_FIG