Discussion of
Miao, an
In Vitro study showing that the antiviral compounds bafilomycin a1 (BafA1), ammonium chloride (NH4Cl), chloroquine (CQ), hydroxychloroquine (HCQ), and dynasore trap SARS-CoV-2 on the cell surface, preventing viral entry through the endocytic pathway. Using live cell imaging with organic dye probes attached to the host receptor ACE2 and viral spike protein, authors track the viral entry pathway from the cell surface to the late endosome. Treatment with 100 nM BafA1 trapped the virus on the cell surface, with almost no virus detected in late endosomes. Other compounds including HCQ also significantly reduced the endosomal to surface virus ratio, suggesting they block endocytic viral entry.
Antiviral compounds like BafA1, NH4Cl, CQ, and HCQ were thought to trap SARS-CoV-2 in late endosomes through alkalinization mechanisms that block a critical proteolytic step needed for viral entry. However, these results suggest that alkalinization of the endosome may not be the only or primary mechanism. It is possible that both of these mechanims play a role:
- Blocking ACE2 endocytosis: the imaging data shows that compounds like HCQ can trap the virus on the cell surface by preventing the internalization of the ACE2 receptor. This suggests that inhibiting viral entry at an early stage is an important mechanism of action for these drugs.
- Endosomal alkalinization: prior studies have shown that compounds like BafA1, NH4Cl, CQ, and HCQ can raise the pH of endosomes, which is thought to inhibit the proteolytic processing needed for SARS-CoV-2 to fuse with the endosomal membrane and release its contents into the cell. The imaging data suggests this may not be the primary mechanism, however it could still play a role in inhibiting any virus particles that manage to enter endosomes, or in different cells/environments where the previous mechanism is less effective.
Note that authors did not perform dose response analysis and the single dose tested for HCQ/CQ is relatively high. Only a small percentage of patients in
Ruiz et al. had ELF concentrations exceeding this dose for 400mg HCQ daily. The new mechanism of action here may require higher concentrations.
Hansen et al., 31 Mar 2023, peer-reviewed, 2 authors.
In Vitro studies are an important part of preclinical research, however results may be very different in vivo.
Getting in on the action: New tools to see SARS-CoV-2 infect a cell
Scott B Hansen, Zixuan Yuan
Cell Chemical Biology, doi:10.1016/j.chembiol.2023.02.010
In this issue of Cell Chemical Biology, Miao et al. develop probes for live cell tracking of SARS-CoV-2. The probes reveal the endocytic pathway for viral entry. Unexpectedly, the antiviral compound BafA1 traps the virus on the cell surface, highlighting the power of super-resolution imaging in live cells.
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