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Rab GTPases not only play a crucial role in the exocytosis and membrane transport of all eukaryotic cells, but are also gradually being found to be involved in innate immunity in mammals. However, the immunological function of Rab3a in fish remains unclear. In this study, two members of Rab3a, CiRab3aa and CiRab3ab, were cloned from grass carp (Ctenopharyngodon idella) for the first time. It was observed that CiRab3aa exhibited a prompt response to grass carp reovirus (GCRV) infection, and suppressing the expression of the CiRab3aa effectively inhibited GCRV replication, whereas the expression level of the CiRab3ab gene did not affect GCRV replication. Furthermore, overexpression of CiRab3aa significantly suppressed the mRNA levels of TLR9 , TNF-α , IL-1β , IL-6 , IL-10 , ERK , and IFN-I following stimulation with CpG ODN 1670 A, suggesting that CiRab3aa is a negative regulator of TLR9 signaling pathway. In conclusion, this study not only revealed functional differences between CiRab3aa and CiRab3ab in grass carp immunity, providing new sights into understanding their function in teleost fish, but also provided a new target for the treatment strategy of grass carp hemorrhagic disease. These results will help address the harm and economic losses caused by GCRV infection in grass carp breeding industry. • CiRab3aa and CiRab3ab genes were cloned and characterized in grass carp. • There are 7 amino acid differences in the Rab domain of Rab3aa and Rab3aa protein. • Knockdown of CiRab3aa significantly inhibited the replication of GCRV. • CiRab3aa was a negative regulator of the TLR9 signaling pathway. [ABSTRACT FROM AUTHOR]

Abstract: Dicer catalyzes the initiation step of RNA interference (RNAi) which is known to play a significant role in innate immune response to viral infection in many organisms. To study the RNAi-related pathway after virus infection in fish, we identified a partial cDNA sequence of dicer from rare minnow, Gobiocypris rarus. Real-time quantitative RT-PCR (qRT-PCR) demonstrated the Dicer transcript level was the highest at zygote stage, decreased at prim-5 stage, and was stable from the protruding mouth to adult stage. Regular RT-PCR analysis showed that the Dicer gene expressed widely in the tested tissues, including brain, gill, heart, intestine, kidney, liver, muscle, ovary, spleen and testis. The expression of Dicer mRNA was significantly increased in the early period of Grass carp reovirus (GCRV) infection, and declined from 24 h post-injection (h p.i.) (P <0.05). The mRNA expression returned to control levels at 48 h p.i. (P >0.05). Under transmission electron microscope, virions were difficulty to find out in 12 h p.i., and virus inclusion bodies and few scattered viral particles were easily visualized from 24 h p.i. to moribund. These results implied GCRV triggered the RNAi pathway in the early stages of infection and perhaps virus inclusion bodies suppressed the antiviral functions of RNAi mechanism. [Copyright &y& Elsevier]