Micro-Transcriptome Analysis Reveals Immune-Related MicroRNA Regulatory Networks ofParalichthys olivaceusInduced byVibrio anguillarumInfection

MicroRNAs (miRNAs) are non-coding regulatory RNAs that play a vital part in the host immune response to pathogen infection. Japanese flounder (Paralichthys olivaceus) is an important aquaculture fish species that has suffered from bacterial diseases, including that caused byVibrio anguillaruminfection. In a previous study, we examined the messenger RNA (mRNA) expression profiles of flounder duringV. anguillaruminfection and identified 26 hub genes in the flounder immune response. In this study, we performed the micro-transcriptome analysis of flounder spleen in response toV. anguillaruminfection at 3 different time points. Approximately 277 million reads were obtained, from which 1218 miRNAs were identified, including 740 known miRNAs and 478 novel miRNAs. Among the miRNAs, 206 were differentially expressed miRNAs (DEmiRs), and 104 of the 206 DEmiRs are novel miRNAs identified for the first time. Most of the DEmiRs were strongly time-dependent. A total of 1355 putative target genes of the DEmiRs (named DETGs) were identified based on integrated analysis of miRNA-mRNA expressions. The DETGs were enriched in multiple functional categories associated with immunity. Thirteen key DEmiRs and 66 immune DETGs formed an intricate regulatory network constituting 106 pairs of miRNAs and DETGs that span five immune pathways. Furthermore, seven of the previously identified hub genes were found to be targeted by 73 DEmiRs, and together they formed interlinking regulatory networks. These results indicate thatV. anguillaruminfection induces complicated miRNA response with extensive influences on immune gene expression in Japanese flounder.