Antiviral activity of the coumarin derivative scoparone against viral hemorrhagic septicemia virus in vitro and in the olive flounder Paralichthys olivaceus

Abstract

Viral hemorrhagic septicemia virus (VHSV) is one of the most important viruses in the aquaculture of farmed finfish. As there is no single solution to prevent or treat VHSV infection, the role of chemical drugs as candidate antiviral agents is indispensable. In this study, we screened 16 coumarin derivatives against VHSV using fathead minnow (FHM) cells by evaluating the cytotoxicity, the inhibition of the VHSV cytopathic effect (CPE) and VHSV N gene expression, respectively. Scoparone (6,7-dimethoxycoumarin), stood out as the most effective compound against VHSV, showing 103.5 ± 10.2 μg/mL 20% cytotoxicity (CC20), 40% CPE reduction and 9.8-fold (P < 0.001) VHSV N gene inhibition at 100 μg/mL. Scoparone, at 100 μg/mL inhibited the VHSV infection by 1-h postinfection treatment, with 52.0% ± 17.1% plaque reduction (P < 0.05), and 90.6% VHSV N gene inhibition (P < 0.05) indicating it inhibits viral internalization and replication. Scoparone exhibited direct virucidal activity, increasing with exposure period, up to 32.3% ± 1.3% plaque reduction from 4 h incubation (P < 0.001), indicating its reversible nature. Scoparone-medicated feed protected the fish from VHSV infection, with up to 44% relative percent survival by 1 mg (long-term) and 10 mg (short-term) prophylactic administration per kg body weight (bw) per day for 21 and 7 days, respectively, in the olive flounder Paralichthys olivaceus. No negative effects were noted on the growth, biochemical, or innate immunological parameters of the fish administered up to 3 mg/kg bw/day of scoparone-medicated feed for 28 days. The administration of scoparone-medicated feed at 1 mg/kg bw/day for 14 days and subsequent VHSV challenge decreased VHSV N gene expression by 48 h postinfection (hpi) in olive flounders. Increased expression levels of IFN I and ISG15 suggested the pivotal role of the IFN I system in conferring scoparone-mediated protection against VHSV infection. The activation of MDA5 at 48 hpi must be attributed to the elevated interferon activity. Elevated interferon type II expression levels suggested the induction of adaptive immune responses. The expression of TLR2 was suppressed at 72 hpi, reflecting reduced viral replication by scoparone. The suppressed expression of caspase 3 suggested that scoparone inhibits virus-induced apoptosis. In summary, scoparone can be considered a practical candidate for protecting farmed finfish against VHSV. It can protect olive flounders from VHSV infection through multiple mechanisms of action and by a convenient mode of administration via medicated feed.