Targeting Stem-loop 1 of the SARS-CoV-2 5'UTR to suppress viral translation and Nsp1 evasion
Setu M. VoraiD, Pietro FontanaiD, Tianyang MaoiD, Valerie Leger, Ying ZhangiD, Tian‐Min FuiD, Judy LiebermaniD, Lee GehrkeiD, Ming ShiiD, Longfei WangiD, Akiko IwasakiiD, Hao WuiD
Abstract
SARS-CoV-2 is a highly pathogenic virus that evades anti-viral immunity by interfering with host protein synthesis, mRNA stability, and protein trafficking. The SARS-CoV-2 nonstructural protein 1 (Nsp1) uses its C-terminal domain to block the mRNA entry channel of the 40S ribosome to inhibit host protein synthesis. However, how SARS-CoV-2 circumvents Nsp1-mediated suppression for viral protein synthesis and if the mechanism can be targeted therapeutically remain unclear. Here we show that N- and C-terminal domains of Nsp1 coordinate to drive a tuned ratio of viral to host translation, likely to maintain a certain level of host fitness while maximizing replication. We reveal that the SL1 region of the SARS-CoV-2 5 UTR is necessary and sufficient to evade Nsp1-mediated translational suppression. Targeting SL1 with locked nucleic acid antisense oligonucleotides (ASOs) inhibits viral translation and makes SARS-CoV-2 5 UTR vulnerable to Nsp1 suppression, hindering viral replication in vitro at a nanomolar concentration. Thus, SL1 allows Nsp1 to switch infected cells from host to SARS-CoV-2 translation, presenting a therapeutic target against COVID-19 that is conserved among immune-evasive variants. This unique strategy of unleashing a virus own virulence mechanism against itself could force a critical trade off between drug resistance and pathogenicity.
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In-depth summary
Claude Sonnet 4.6 · xhigh effort · 7 Nov 2026 · from the full text of version 1 (9 Sept 2021) · CC BY 4.0
SARS-CoV-2 5' UTR stem-loop 1 (SL1) lets reporters escape Nsp1 translational shutdown. LNA antisense oligos against SL1 restore Nsp1 suppression and reduce infected Vero E6 cells.
It was unclear how SARS-CoV-2 gets around Nsp1-mediated suppression for its own protein synthesis, and whether that route could be a drug target. The paper maps the bypass to SL1, shows that both Nsp1 domains are needed, and tests SL1-directed antisense oligonucleotides (ASOs).
Key results
ShownIn HeLa reporter assays, the SARS-CoV-2 5' UTR escapes Nsp1 suppression, while control and human MAVS 5' UTRs are strongly suppressed.
mScarlet fell by over 7.1-fold (control 5' UTR) and 8.1-fold (MAVS 5' UTR) with MBP-Nsp1 vs MBP (p<0.001). The SARS-CoV-2 5' UTR reporter showed no significant change. ORF3a-GFP was not significantly decreased; ORF8-GFP fell 1.4-fold. · SARS-CoV-2 5’ UTR mediates translation despite…
ShownSL1 is necessary and sufficient for escaping Nsp1 suppression in this reporter system.
A SARS-CoV-2 5' UTR lacking SL1 (ΔSL1) was reduced 6.3-fold by Nsp1 (p<0.001), similar to control and MAVS reporters. A reporter with SL1 alone was not significantly reduced. HeLa cells, mScarlet imaging. · The SL1 of the 5’ UTR is necessary and…
ShownBoth Nsp1 N-terminal (NT) and C-terminal (CT) domains are needed for host shutdown and for the viral bypass. The viral-to-host reporter ratio is the readout that stays consistent across cell lines.
Full-length Nsp1 gave a CoV-2/control fluorescence ratio of 5.6 ± 1.2 (HeLa) and 5.9 ± 0.62 (293T). NT or CT alone gave ~10-fold lower ratios. NT+CT from separate constructs also lowered it. Adding 20 or 40 linker residues sharply reduced the ratio (p<0.001). A luciferase reporter in 293T agreed. · Nsp1-CT and Nsp1-NT are both required for…; Correct association and spacing of NT+CT via the…
ShownLNA ASOs against SL1 suppress the SARS-CoV-2 5' UTR reporter. ASO4 and ASO7 also synergize with Nsp1, whereas ASO6 acts alone. Short (≤15 base) LNAs and DNA or RNA ASOs were inactive at 50 nM.
In HeLa cells at 50 nM, ASO4 (24 base LNA) suppressed the reporter alone and more with Nsp1; with Nsp1 it fell below the ΔSL1 reporter. ASO6 and ASO7 also suppressed the reporter and showed little activity on ΔSL1. Dose-response was tested at 25, 50 and 100 nM. · SL1 antisense oligos (ASO) selectively target…
SuggestedAnti-SL1 LNA ASOs reduced the share of infected cells among transfected Vero E6 cells.
At 100 nM and MOI 0.1 or 0.5, nucleocapsid-positive mScarlet-positive cells fell ~4-fold with ASO4 and ASO6 and ~2-fold with ASO7 vs control LNA. Cells were read out at 72 h. One panel used two replicates. · SL1 antisense oligos effectively inhibit…; Quantification and statistical analysis; In vitro infection with SARS-CoV-2
Methods, data and reproducibility
Fluorescent (mScarlet) and luciferase reporters carrying different 5' UTRs were co-expressed with Nsp1 variants (full length, NT, CT, linker and point mutants) in cell lines and quantified by imaging. LNA ASOs against SL1 were screened in the same reporters, then tested in infected Vero E6 cells.
- Reporter assays with Nsp1 variants and ASOs in HeLa cells (imaging) · human (HeLa cell line) · n = at least 3 replicate wells per treatment; 2-9 imaging sites per well
- Reporter assays in HEK293T cells (imaging and dual luciferase) · human (HEK293T cell line) · n = at least 3 replicates; luciferase pooled from 2 wells, 3 measurements each
- ASO reporter time course and SARS-CoV-2 infection in Vero E6 cells (MOI 0.1 and 0.5) · n = duplicate in 24-well plates for Fig. 5E; two replicates for Fig. 5D
- Methods & tools
- mScarlet reporter, Dual Luciferase Assay System, LNA mixmer ASOs, ImageJ, MetaXpress, GraphPad Prism 5, ImageXpress Micro Confocal
- Reagents & environment
- GraphPad Prism 5 for statistics; ImageJ or MetaXpress for image quantification.
- Reproducibility gaps
- No code or data repository links are given. The Nsp1-SL1 binding test is reported as 'data not shown'. ASO sequences are shown only in a schematic figure.
Limitations (3)
- The molecular basis of how Nsp1 NT works with SL1 to bypass suppression is unresolved. The authors could not detect direct Nsp1-SL1 binding under physiological salt (data not shown) and suggest indirect interaction. · stated by the authors
- Evidence comes from transient-transfection reporter assays with Nsp1 supplied from plasmids. Antiviral activity was tested only in Vero E6 cells at 100 nM, with duplicate replicates in some panels. No dose-response in infection and no in vivo data are shown. · noted in this summary
- The argument that resistance to anti-SL1 ASOs would be evolutionarily constrained (co-mutation of SL1 and Nsp1 needed) is not tested experimentally here. · noted in this summary
Who should read:Read this if you work on coronavirus translation control, Nsp1 mechanism, or antisense strategies that target viral 5' UTR elements.
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