Viral nervous necrosis (VNN) is an acute and serious fish disease caused by nervous necrosis virus (NNV) which has been reported massive mortality in more than fifty teleost species worldwide. VNN causes damage of necrosis and vacuolation to central nervous system (CNS) cells in fish. It is difficult to identify the specific type of cell targeted by NNV, and to decipher the host immune response because of the functional diversity and highly complex anatomical and cellular composition of the CNS. In this study, we found that the red spotted grouper NNV (RGNNV) mainly attacked the midbrain of orange-spotted grouper (Epinephelus coioides). We conducted single-cell RNA-seq analysis of the midbrain of healthy and RGNNV-infected fish and identified 35 transcriptionally distinct cell subtypes, including 28 neuronal and 7 non-neuronal cell types. An evaluation of the subpopulations of immune cells revealed that macrophages were enriched in RGNNV-infected fish, and the transcriptional profiles of macrophages indicated an acute cytokine and inflammatory response. Unsupervised pseudotime analysis of immune cells showed that microglia transformed into M1-type activated macrophages to produce cytokines to reduce the damage to nerve tissue caused by the virus. We also found that RGNNV targeted neuronal cell types was GLU1 and GLU3, and we found that the key genes and pathways by which causes cell cytoplasmic vacuoles and autophagy significant enrichment, this may be the major route viruses cause cell death. These data provided a comprehensive transcriptional perspective of the grouper midbrain and the basis for further research on how viruses infect the teleost CNS.

Iridovirus has become a great threat to global aquaculture due to its high mortality, while the molecular events in virus pathogenesis are not well elucidated. Here, the multi-omics approach was performed with Singapore grouper iridovirus (SGIV)-infected groupers, and the roles of key metabolites were investigated. SGIV induced obvious histopathological damage and changes of metabolic enzymes in liver. Besides, SGIV significantly decreased the contents of lipid droplets, triglycerides, cholesterol, and lipoprotein. Metabolome analysis showed that that the differentially metabolites were enriched in 19 pathways, and lipid metabolites, including glycerophosphates and alpha-linolenic acid (ALA) were down-regulated, which is consistent with the disturbed lipid homeostasis in liver. Combining with transcriptomic and metabolomic data, the top enriched pathways were related to cell growth and death, nucleotide, carbohydrate, amino acid, and lipid metabolisms, supporting the conclusion that SGIV infection induced liver metabolic reprogramming. Integrative transcriptomic and proteomic analysis indicated that SGIV infection activated the crucial molecular events in phagosome-immune depression-metabolism dysregulation-necrosis signaling cascade. Of note, integrative multi-omics analysis demonstrated that metabolites ALA and linoleic acid (LA) were consumed, while L-glutamic acid (GA) was accumulated, accompanied by the alteration of immune, inflammation, and cell death related genes. Further experimental data showed that ALA, but not GA suppressed SGIV replication by activating host antioxidant and anti-inflammatory effects. Together, our findings for the first time provide a comprehensive resource to understand the landscape of host response dynamics during fish iridovirus infection, and highlight the antiviral roles of ALA in the prevention and treatment for iridovirus diseases.

Autophagy is an important biological activity that maintains homeostasis in eukaryotic cells. However, little is known about the functions of fish autophagy-related genes (Atgs). In this study, we cloned and characterized Atg5, a key gene in the autophagy gene superfamily, from orange-spotted grouper (Epinephelus coioides) (EcAtg5). EcAtg5 encoded a 275-amino acid protein that shared 94% and 81% identity to seabass (Lates calcarifer) and humans (Homo sapiens), respectively. The transcription level of EcAtg5 was significantly increased in cells infected with red-spotted grouper nervous necrosis virus (RGNNV). In cells infected with Singapore grouper iridovirus (SGIV), EcAtg5 expression declined during the early stage of infection and increased in the late stage. Fluorescence microscopy revealed that EcAtg5 mainly localized with a dot-like pattern in the cytoplasm of grouper cells. Overexpression of EcAtg5 significantly increased the replication of RGNNV and SGIV at different levels of detection, as indicated by increased severity of the cytopathic effect, transcription levels of viral genes, and levels of viral proteins. Knockdown of EcAtg5 decreased the replication of RGNNV and SGIV. Further studies showed that overexpression EcAtg5 activated autophagy, decreased expression levels of interferon related cytokines or effectors and pro-inflammatory factors, and inhibited the activation of nuclear factor B, IFN-sensitive response element, and IFNs. In addition, ectopic expression of EcAtg5 affected cell cycle progression by hindering the G1/S transition. Taken together, our results demonstrated that fish Atg5 exerted a crucial role in virus replication by promoting autophagy, down-regulating antiviral IFN responses, and affecting the cell cycle.