Sequence structure and immune function of signal transduction and transcriptional activator STAT1 of barbel chub (Squaliobarbus curriculus)

The grass carp hemorrhage disease poses a serious threat to the long-term expansion of the aquaculture industry. The barbel chub (Squaliobarbus curriculus) could hybrid with the grass carp to produce progeny possessing resistance to GCRV infection and, therefore, are considered valuable genetic resources for studying the molecular mechanisms of grass carp hemorrhage. To investigate the immune function of barbel chub STAT1 (ScSTAT1) against GCRV infection, the RACE (rapid amplification of cDNA ends), qPCR (quantitative polymerase chain reaction) and RNAi (RNA interference) techniques were applied to obtain the full-length cDNA sequence of ScSTAT1, to detect its expression profile in healthy and GCRV-infected tissues, and to explore its basic immune function. The ScSTAT1 was 2 922 bp in length and encoded a protein of 718 amino acid residues. The ScSTAT1 contained conserved domains for STAT_int, STAT_alpha, STAT_binding and SH2. Phylogenetic analysis revealed that the ScSTAT1 was closely clustered with the homologue from Ctenopharyngodon idella, forming an extended clade with those from Mylopharyngodon piceus, Carassius auratus and Tachysurus fulvidraco. Compared to the homologues from mammals and reptiles, ScSTAT1 lacked a C-terminal TAD domain where a serine phosphorylation site is present. The expression level of ScSTAT1 was shown to be the highest in the spleen among the tissues analyzed, with the lowest level detected in the liver. Treatment of fish with LPS, Poly I:C or GCRV resulted in upregulation of expressions of ScSTAT1 in the spleen and kidney. At 12 h post GCRV infection, the ScSTAT1 expression was down-regulated in the spleen, followed by increases. At 12 h and 72 h post GCRV infection, in trunk kidney the expression levels of ScSTAT1 were significantly higher than those in the control group. In the S. curriculus fin cell (ScF) line, knockdown 60% of ScSTAT1 expression by RNA interference led to decreased expression of IRF3, IRF9 and Mx at 48 h post GCRV infection. The results of the present study proved that ScSTAT1 participated in the signal transduction of the IFN system and played a key role in immune reaction against GCRV infection. The results also provide research basis for further study on the functions of ScSTAT1 in disease resistance in fish.