Identification and functional analysis of dual-specificity MAP kinase phosphatase 6 gene (dusp6) in response to immune challenges in Japanese flounder Paralichthys olivaceus

Dual-specificity phosphatase 6 (Dusp6) is a member of mitogen-activated protein kinase (MAPK) phosphatases that play crucial roles in regulating MAPK signaling and immune response. The immunological relevance of Dusp6 in fish, however, remains largely uncharacterized. In the present study, a full-length Japanese flounder dusp6 cDNA ortholog, termed PoDusp6, was identified and characterized from Paralichthys olivaceus. The deduced PoDusp6 protein is comprised of 383 amino acids with a conserved N-terminal regulatory rhodanese homology domain and a C-terminal catalytic domain. Immunofluorescence microscopy revealed that PoDusp6 protein is mainly localized in cytoplasm. Sequence analysis indicates that PoDusp6 is highly conserved (>70% identity) throughout the evolution from teleost to mammals. In unstimulated conditions, PoDusp6 mRNA was present in all examined tissues and showed the highest expression in Japanese flounder head kidney macrophages (HKMs). Immune challenge experiments revealed that the expression of PoDusp6 was down-regulated at the early stage after LPS and poly(I:C) stimulations but significantly up-regulated at the later stage in the HKMs. The similar expression pattern was also observed in the Japanese flounder immune-related tissues including head kidney, gill and spleen upon bacterial challenge with Edwardsiella tarda. Overexpression of PoDusp6 in Japanese flounder FG-9307 cells led to a significant down-regulation of proinflammatory cytokine genes IL-1beta, TNF-alpha and IFN-gamma, and antiviral gene Mx. Interestingly, inhibition of Dusp6 activity also down-regulated the LPS-induced IL-beta gene expression but did not affected on the LPS-induced IFN-gamma and TNF-alpha expression in the HKMs. Our findings suggest that the expression of PoDusp6 is modulated by immune stimuli and PoDusp6 may act as an essential modulator in fish inflammatory response.