White spot syndrome virus-encoded microRNA promotes viral replication by maintaining viral early gene expression

Disease especially caused by White spot syndrome virus (WSSV) has a huge impact on the shrimp production industry. An understanding of shrimp-WSSV interaction could lead to an effective approach to control WSSV infection. MicroRNA (miRNA) is a short small RNA which functions in post-transcriptional gene regulation and is involved in the WSSV infection. Previously, WSSV-encoded miRNAs were identified from infected P. monodon. We investigated wsv-miR-9 which showed the highest expression in WSSV-infected gill tissue. The expression of wsv-miR-9 was up-regulated upon WSSV infection and reduced in moribund shrimp. Antisense inhibition using anti-miRNA oligonucleotides (AMO) resulted in a complete inhibition of wsv-miR-9 expression. An in vitro functional study demonstrated that mimic of wsv-miR-9 could function in translational suppression by decreasing the reporter protein activity when compared to the mutant target. In shrimp, loss of wsv-miR-9 by AMO9 resulted in a delay of viral replication after 6–12 h post WSSV challenge. A reduction of WSSV early genes, ie1 and DNA polymerase mRNA expressions was observed within 6 h. These results reveal that wsv-miR-9 is important in maintaining early viral gene expression.