Thapsigargin: key to new host-directed coronavirus antivirals?

Abstract

Despite the great success of vaccines that protect against RNA virus infections, and the development and clinical use of a limited number of RNA virus-specific drugs, there is still an urgent need for new classes of antiviral drugs against circulating or emerging RNA viruses. To date, it has proved difficult to efficiently suppress RNA virus replication by targeting host cell functions, and there are no approved drugs of this type. This opinion article discusses the recent discovery of a pronounced and sustained antiviral activity of the plant-derived natural compound thapsigargin against enveloped RNA viruses such as severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), Middle East respiratory syndrome coronavirus (MERS-CoV), and influenza A virus. Based on its mechanisms of action, thapsigargin represents a new prototype of compounds with multimodal host-directed antiviral activity.

Keywords: COVID-19; ER membranes; ERAD; coronavirus; host-directed antivirals; proteome; thapsigargin.

Figure 1

Key figure. Hypothetical antiviral mechanism of thapsigargin involving endoplasmic reticulum (ER)-associated protein degradation (ERAD). The scheme shows the organization of the ER membrane-associated ERAD complex as recently reviewed [30]. ERAD is intimately linked to the major proteasomal and autophagy-related protein degradation pathways [70., 71., 72.] (Box 1). Red colors highlight the proteins endoplasmic reticulum chaperone binding immunoglobulin protein (BiP; GRP-78, HSPA5), homocysteine-responsive endoplasmic reticulum-resident ubiquitin-like domain member 1 (HERPUD1), ubiquitin-like modifier-activating enzyme 6 (UBA6), autophagy-related protein LC3 (microtubule-associated proteins 1A/1B light chain 3A/B), and ubiquitin-binding protein p62/sequestosome-1 (p62/SQSTM1) that, by mass spectrometry and immunoblotting, have consistently been found to be induced by thapsigargin in coronavirus (CoV)-infected cells. Question marks indicate potential functional links between these factors. Because thapsigargin-treated infected cells show a strong reduction in CoV protein levels, it is tempting to suggest that thapsigargin enhances ERAD activity in favor of autophagy (which is blocked by the compound late in the autophagy pathway) and, thereby, causes disposal of viral proteins primarily through the proteasome pathway as part of its antiviral mechanism. Abbreviations: DERL1, Derlin-1; FAT10, a ubiquitin-like protein (ubiquitin D); HRD1, E3 ubiquitin-protein ligase synoviolin; SEL1L, protein sel-1 homolog 1; VCP, transitional endoplasmic reticulum ATPase, 5S Mg²⁺-ATPase p97 subunit, valosin-containing protein.

Figure I

Scheme and structure models of SERCA with the Ca²⁺ and thapsigargin (Tg) binding sites indicated by circles. Reproduced and adapted from [78] under a Creative Commons Attribution Non Commercial License 4.0 (CC BY-NC). Abbreviations: AMPPCP, adenylyl methylenediphosphonate; SERCA, sarco-/endoplasmic reticulum (ER) Ca²⁺-ATPase.

Figure 2

Summary of thapsigargin effects in coronavirus (CoV)-infected cells and possible modes of application in humans. (A) The scheme shows the different levels of cellular processes that have been shown to be activated or suppressed by thapsigargin in cells infected with various CoVs. Also included are the effects of drugs blocking the lysosome (bafilomycin A₁) or PERK kinase (PERKi, GSK2656157 or GSK2606414). At present, the role of thapsigargin-mediated calcium depletion in coronavirus replication is unclear. Adapted from [15]. Abbreviations: Tg, thapsigargin; ER, endoplasmic reticulum; ERQC, ER quality control; ERAD, ER-associated protein degradation; PERK, protein kinase RNA-activated (PRKR)-like endoplasmic reticulum kinase; UPR, unfolded protein response. (B) Possible routes and modes of application of thapsigargin in humans. While short-term oral administration was effective in mice, the bioavailability and pharmacokinetics of thapsigargin for the use as antiviral therapy in humans are currently unclear. (C) Structure of thapsigargin. (D) Potential protease-cleavable prodrugs in which thapsigargin is coupled to a peptide (in green) that will be cleaved off by cell type-specific proteases, by (inducible) proteases present only in infected cells, or by coronavirus proteases, in order to minimize side effects by restricting the active drug primarily or exclusively to infected cells.