Emodin enhances host antiviral immunity against Micropterus salmoides rhabdovirus by activating RLR signaling in largemouth bass

Micropterus salmoides rhabdovirus (MSRV) is a highly pathogenic virus that causes substantial mortality in largemouth bass, yet effective Antiviral agents remain unavailable. In this study, we evaluated the Antiviral potential of 20 plant-derived small molecules and identified emodin as the most effective inhibitor. At 25 mg/L, emodin reduced MSRV nucleoprotein gene expression in epithelioma papulosum cyprinid (EPC) cells by 93 %. Time-course analysis revealed a strong prophylactic effect, with viral inhibition increasing with longer pretreatment durations and peaking at 24 h (87 % inhibition). In contrast, its therapeutic effect diminished as Infection progressed and was no longer effective at 24 h post-infection. Emodin significantly protected host cells from virus-induced damage, reducing Apoptosis by 69 % as determined by flow cytometry. 4',6-diamidino-2-phenylindole (DAPI) and Mito-Tracker staining confirmed that emodin mitigated nuclear condensation and restored mitochondrial morphology. Mechanistically, emodin inhibited the MSRV-induced upregulation of the dynamin-related protein 1 (Drp1) while restoring mitofusin 2 (Mfn2) levels, thereby maintaining mitochondrial integrity. These changes were accompanied by activation of the mitochondria Antiviral signaling protein (MAVS) pathway and elevated expression of immune-related genes. Reverse transcription quantitative PCR (RT-qPCR) showed that emodin increased the expression of interferon regulatory factor 7(irf7), tank-binding kinase 1(TBK1), and retinoic acid-inducible gene i (rig-i) by 4.38-, 2.95-, and 1.9-fold, respectively. In vivo, dietary administration of emodin (1 %) for 15 days significantly upregulated interferon regulatory factor 3(irf3), irf7, and interferon gamma (ifnγ) expression in liver and spleen. Upon MSRV challenge, fish in the 1 % emodin group exhibited 80 % and 82 % reductions in viral gene expression in liver and spleen, respectively, and their cumulative survival rate increased by 26 %. Collectively, these findings demonstrate that emodin confers Antiviral protection by preserving mitochondrial homeostasis and activating innate immune responses, highlighting its potential as an effective and practical Antiviral agent in largemouth bass aquaculture.

Active molecules; Antiviral; Apoptosis; Innate immune system; Micropterus salmoides rhabdovirus.