Thiopurines inhibit coronavirus Spike protein processing and incorporation into progeny virions
Bibliographic record
Abstract
ABSTRACT There is an outstanding need for broadly acting antiviral drugs to combat emerging viral diseases. Here, we report that thiopurines inhibit the replication of the betacoronaviruses HCoV-OC43 and SARS-CoV-2, and to a lesser extent, the alphacoronavirus HCoV-229E. 6-Thioguanine (6-TG) disrupted early stages of infection, limiting synthesis of full-length and subgenomic HCoV RNAs. Furthermore, consistent with our previous report on the effects of thiopurines on influenza A virus glycoproteins, we observed that 6-TG inhibited accumulation of Spike glycoproteins from diverse HCoVs. Specifically, 6-TG treatment decreased the accumulation of Spike proteins and increased their electrophoretic mobility, consistent with Spike migration following the enzymatic removal of N-linked oligosaccharides with Peptide:N-glycosidase F (PNGaseF). SARS-CoV-2 virus-like particles (VLPs) harvested from 6-TG-treated cells were deficient in Spike. 6-TG treatment had a similar effect on lentiviruses pseudotyped with SARS-CoV-2 Spike; lentiviruses could be harvested from cell supernatants but were deficient in Spike and unable to infect human cells bearing ACE2 receptors. Together, these findings from complementary ectopic expression and infection models strongly indicate that defective Spike trafficking and processing is an outcome of 6-TG treatment. At low micromolar doses, the primary known mode of action of 6-TG is selective inhibition of the small GTPase Rac1. However, we observed that selective chemical inhibitors of the small GTPases Rac1, CDC42 and Rho had no effect on Spike processing and accumulation. The GTPase agonist ML099 countered the effects of 6-TG, suggesting that an unknown GTPase could be the relevant 6-TG-target protein involved in regulating Spike processing and accumulation. Overall, these findings provide important clues about the mechanism of action of a candidate antiviral that can broadly target HCoVs and suggest that small GTPases are promising targets for host-targeted antivirals. AUTHOR SUMMARY The COVID-19 pandemic has ignited efforts to repurpose existing drugs as safe and effective antivirals. Rather than directly inhibiting viral enzymes, host-targeted antivirals inhibit host cell processes to indirectly impede viral replication and/or stimulate antiviral responses. Here, we describe a new antiviral mechanism of action for the FDA-approved thiopurine 6-thioguanine. We demonstrate that this thiopurine is a pro-drug that must be metabolized by host enzymes to gain antiviral activity. We show that it can inhibit the replication of several human coronaviruses, including SARS-CoV-2, at least in part by interfering with the processing and accumulation of Spike glycoproteins, thereby impeding assembly of infectious progeny viruses. We provide evidence implicating host cell GTPase enzymes in the antiviral mechanism of action.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".