Beta-nodavirus B2 protein induces hydrogen peroxide production, leading to Drp1-recruited mitochondrial fragmentation and cell death via mitochondrial targeting

Because the role of the viral B2 protein in the pathogenesis of nervous necrosis virus infection remains unknown, the aim of the present study was to determine the effects of B2 protein on hydrogen peroxide (H₂O₂)-mediated cell death via mitochondrial targeting. Using a B2 deletion mutant, the B2 mitochondrial targeting signal sequence (⁴¹RTFVISAHAA⁵⁰) correlated with mitochondrial free radical production and cell death in fish cells, embryonic zebrafish, and human cancer cells. After treatment of grouper fin cells (GF-1) overexpressing B2 protein with the anti-oxidant drug, N-acetylcysteine (NAC), and overexpression of the antioxidant enzymes, zfCu/Zn superoxide dismutase (SOD) and zfCatalase, decreased H₂O₂ production and cell death were observed. To investigate the correlation between B2 cytotoxicity and H₂O₂ production in vivo, B2 was injected into zebrafish embryos. Cell damage, as assessed by the acridine orange assay, gradually increased over 24 h post-fertilization, and was accompanied by marked increases in H₂O₂ production and embryonic death. Increased oxidative stress, as evidenced by the up-regulation of Mn SOD, catalase, and Nrf2, was also observed during this period. Finally, B2-induced dynamin-related protein 1 (Drp1)-mediated mitochondrial fragmentation and cell death could be reversed by NAC and inhibitors of Drp1 and Mdivi in GF-1 cells. Taken together, betanodavirus B2 induces H₂O₂ production via targeting the mitochondria, where it inhibits complex II function. H₂O₂ activates Drp1, resulting in its association with the mitochondria, mitochondrial fission and cell death in vitro and in vivo.