Identification and characterization of ATP-gated P2X2 receptor gene dominantly expressed in the Japanese flounder (Paralichthys olivaceus) head kidney macrophages

Activated P2X receptors can up-regulate the expressions of inflammation-related genes via NF-κB pathway in spotted sea bass (Lateolabrax maculatus)

P2X receptors, including seven subtypes, i.e., P2X1-7, are the ligand-gated ion channels activated by the extracellular ATP playing the critical roles in inflammation and immune response. Even though the immune functions of P2X receptors have been characterized extensively in mammals, their functions in fish remain largely unknown. In this study, four P2X receptor homologues were characterized in spotted sea bass (Lateolabrax maculatus), which were named LmP2X2, LmP2X4, LmP2X5, and LmP2X7. Their tissue distributions and expression patterns were then investigated by real-time quantitative PCR (qPCR). Furthermore, their functions in regulating the expressions of inflammation-associated genes and possible signaling pathway were examined by qPCR and luciferase assay. The results showed that they share similar topological structures, conserved genomic organization, and gene synteny with their counterparts in other species previously investigated. And the four P2X receptors were expressed constitutively in the tested tissues. In addition, the expression of each of the four receptor genes was significantly induced by stimulation of Edwardsiella tarda and/or pathogen-associated molecular patterns (PAMPs) in vivo. Also, in primary head kidney leukocytes of spotted sea bass, LmP2X2 and LmP2X5 were induced by using PAMPs and/or ATP. Notably, the expressions of CCL2, IL-8, and TNF-α recognized as the pro-inflammatory cytokines, and of the four apoptosis-related genes, i.e., caspase3, caspase6, caspase7, and P53, were differentially upregulated in the HEK 293T cells with over-expressed LmP2X2 and/or LmP2X7 following ATP stimulation. Also, the over-expression of LmP2X4 can upregulate the expressions of IL-8, caspase6, caspase7, and P53, and LmP2X5 upregulates of IL-8, TNF-α, caspase7, and P53. Then in the present study it was demonstrated that the activation of any one of the four receptors significantly upregulated the activity of NF-κB promoter, suggesting that the activated LmP2Xs may regulate the expressions of pro-inflammatory cytokines via the NF-κB pathway. Taken together, the four P2X receptors were identified firstly from fish species in Perciformes, and they participate in innate immune response of spotted sea bass possibly by regulating the expressions of the inflammation-related genes. Our study provides the new evidences for the P2X receptors' involvement in fish immunity.

Extracellular ATP is a potent signaling molecule in the activation of the Japanese flounder (Paralichthys olivaceus) innate immune responses

Innate immunity is the first line of defense against pathogen infections. Extracellular ATP (eATP) is one of the most studied danger-associated molecular pattern molecules that can activate host innate immune responses through binding with and activating purinergic receptors on the plasma membrane. The detailed actions of eATP on fish innate immunity, however, remain poorly understood. In this study, we investigated bacterial pathogen-induced ATP release in head kidney cells of the Japanese flounder Paralichthys olivaceus. We also examined the actions of eATP on pro-inflammatory cytokine and immune-related gene expression, the activity of induced NO synthase (iNOS), and the production of reactive oxygen species (ROS) and NO in Japanese flounder immune cells. We demonstrate that ATP is dynamically released from Japanese flounder head kidney cells into the extracellular milieu during immune challenge by formalin-inactivated Edwardsiella tarda and Vibrio anguillarum. In addition, we show that eATP administration results in profound up-regulation of pro-inflammatory cytokine gene expression, iNOS activity, and inflammatory mediator production, including ROS and NO, in Japanese flounder immune cells. Altogether, our findings demonstrate that eATP is a potent signaling molecule for the activation of innate immune responses in fish.

Functional characterization of purinergic receptor P2Y14 in the Japanese flounder (Paralichthys olivaceus) head kidney macrophages

Extracellular nucleotides and nucleotide sugars are important danger-associated signaling molecules that play critical roles in regulation of immune responses in mammals through activation of purinergic receptors located on the cell surface. However, the immunological role of extracellular UDP-glucose-activated P2Y₁₄ receptor (P2Y₁₄R) in fish still remains unknown. In this study, we identified and characterized a P2Y₁₄R paralog in the Japanese flounder (Paralichthys olivaceus). The mRNA transcripts of P2Y₁₄R are detected in all examined Japanese flounder tissues. Compared with the UDP-activated P2Y₆ receptor, however, P2Y₁₄R gene is highly expressed in Japanese flounder head kidney macrophages (HKMs). In addition, P2Y₁₄R is significantly upregulated following inflammatory stimulation with LPS and poly (I:C) in the HKMs, suggesting a role of P2Y₁₄R in response to inflammation in fish. Furthermore, activation of P2Y₁₄ receptor with its potent and selective agonist MRS 2905 resulted in a decreased expression of LPS-induced pro-inflammatory cytokine IL-1beta gene in the HKMs. In contrast, inhibition of P2Y₁₄ receptor activity or down-regulation of the endogenous expression of P2Y₁₄R by small interfering RNA significantly upregulates the LPS-induced pro-inflammatory cytokine IL-1beta gene expression in the HKMs, demonstrating that P2Y₁₄R is involved in inflammation regulation in fish. Moreover, stimulation of the Japanese flounder HKMs with UDP-glucose evoked a rapid increase of extracellular signal-regulated kinase 1/2 (ERK1/2) phosphorylation in a dose- and time-dependent manner, indicating the involvement of P2Y₁₄R in activation of ERK1/2 signaling in fish immune cells. Taken together, we demonstrated that the inducible P2Y₁₄R plays an important role in regulation of fish innate immunity.

Identification and characterization of ecto-nucleoside triphosphate diphosphohydrolase 1 (CD39) involved in regulating extracellular ATP-mediated innate immune responses in Japanese flounder (Paralichthys olivaceus)

Extracellular adenosine triphosphate (eATP), released following inflammatory stimulation or infection, is a potent signaling molecule in activating innate immune responses in fish. However, the regulation of eATP-mediated innate immunity in fish remains unknown. Ecto-nucleoside triphosphate diphosphohydrolase 1 (CD39) is a critical molecular switch for controlling the ATP levels in the extracellular space. CD39 plays a key role in regulating eATP-activated innate immune responses through the phosphohydrolysis of pro-inflammatory eATP to inactive AMP. Here, we identified and characterized a CD39 homolog (namely, poCD39) in the Japanese flounder Paralichthys olivaceus and analyzed its regulatory role in eATP-mediated innate immunity. Real-time quantitative PCR analysis revealed that poCD39 is ubiquitously present in all tested normal tissues with dominant expression in enriched Japanese flounder head kidney macrophages (HKMs). Immune challenge experiments demonstrated that poCD39 expression was upregulated by inflammatory stimulation and Edwardsiella tarda infection. Biochemical and immunofluorescence analysis revealed that poCD39 is a functional glycosylated membrane protein for the hydrolysis of eATP. Inhibition of poCD939 activity with the ecto-NTPDase inhibitor ARL 67156 resulted in increased IL-1beta gene expression and ROS production in Japanese flounder HKMs. In contrast, overexpression of poCD39 in Japanese flounder FG-9307 cells reduced eATP-induced pro-inflammatory cytokine IL-1beta gene expression. Finally, poCD39 expression was significantly induced by eATP stimulation in the HKMs, suggesting that eATP may provide a feedback mechanism for transcriptional regulation of fish CD39. Taken together, we identified and characterized a functional fish CD39 protein involved in regulating eATP-mediated innate immune responses in fish.

Characterization of UDP-Activated Purinergic Receptor P2Y₆ Involved in Japanese Flounder Paralichthys olivaceus Innate Immunity

Uridine 5’-diphosphate (UDP)-activated purinergic receptor P2Y₆ is a member of a G-protein-coupled purinergic receptor family that plays an important role in mammalian innate immunity. However, the role of the P2Y₆ receptor (P2Y₆R) in fish immunity has not been investigated. In this report, we characterized a P2Y₆R gene from Japanese flounder (Paralichthys olivaceus) and examined its role in fish innate immunity. Sequence analysis reveals that the Japanese flounder P2Y₆R protein is conserved and possesses four potential glycosylation sites. Quantitative real-time RT-PCR analysis shows that P2Y₆R is broadly distributed in all examined Japanese flounder tissues with dominant expression in the liver. In addition, P2Y₆R gene expression was up-regulated in head kidney macrophages (HKMs) upon lipopolysaccharides (LPS) and poly(I:C) stimulations but down-regulated by LPS challenge in peripheral blood leukocytes (PBLs). Furthermore, pharmacological inhibition of the endogenous P2Y₆ receptor activity by the potently selective P2Y₆R antagonist, MRS 2578, greatly up-regulated pro-inflammatory cytokine interleukin (IL)-1β, IL-6 and TNF-α gene expression in PBL cells treated with UDP. Moreover, LPS- and poly(I:C)-induced gene expression of IL-1β and TNF-α in Japanese flounder PBL cells was attenuated significantly by inhibition of P2Y₆R activity with antagonist MRS 2578. Collectively, we, for the first time, showed the involvement of functional purinergic P2Y₆R in fish innate immunity.

Functional characterization of purinergic P2Y2 and P2Y12 receptors involved in Japanese flounder (Paralichthys olivaceus) innate immune responses

G-protein-coupled P2Y receptors activated by extracellular nucleotides play important roles under different physiological and pathophysiological conditions in mammals. To investigate the immunological relevance of P2Y receptors in fish, we identified and characterized the P2Y₂ and P2Y₁₂ receptors in Japanese flounder Paralichthys olivaceus. The P. olivaceus P2Y₂ and P2Y₁₂ receptors harbor seven transmembrane domains but share only 24% sequence identity. Real-time PCR analysis revealed the constitutive but unequal mRNA expression pattern of P2Y₂R and P2Y₁₂R in normal Japanese flounder tissues with the dominant expression of P2Y₂R in head kidney and blood and P2Y₁₂R in hepatopancreas. In addition, the expression of P2Y₂ and P2Y₁₂ receptors was markedly modulated by PAMPs stimulation and Edwardsiella tarda infection. Furthermore, blockage of P2Y₁₂R potently increased ADP-activated pro-inflammatory cytokine IL-1beta gene expression in the head kidney macrophages (HKMs). Moreover, inhibition of P2Y₂ and P2Y₁₂ receptor activity with their respective potent antagonists significantly altered some of the LPS-induced pro-inflammatory cytokine gene expression in the HKMs. However, blockade of P2Y₁₂R did not affect the poly(I:C)-induced pro-inflammatory cytokine gene expression examined in the HKMs. Collectively, we have for the first time reported the role of purinergic P2Y₂ and P2Y₁₂ receptors in fish innate immunity. Our findings have also addressed the importance of extracellular ATP and its metabolites in fish innate immune responses.

Divalent metal modulation of Japanese flounder (Paralichthys olivaceus) purinergic P2X7 receptor

Paralichthys olivaceus P2X7 receptor (poP2X7R) is a recently identified as a P2X7 purinergic receptor involved in innate immunity of the Japanese flounder Paralichthys olivaceus. Divalent metals are allosteric modulators of mammalian P2XRs, but there is no information for fish P2XRs. Here, we characterized the effects of divalent metals on poP2X7R channel activity by electrophysiology and molecular biology techniques. Copper, zinc and mercury inhibited poP2X7R‐mediated currents with different maximal inhibition potency, while cadmium had no effect on poP2X7R activity. Mercury‐induced inhibition was irreversible, but the inhibitory effects of copper and zinc were reversed after washout. Cooper and zinc also reduced poP2X7R‐mediated interleukin‐1 mRNA production. These findings suggest that divalent metals have potential effects on the Japanese flounder innate immune response through modulation of poP2X7R activity.

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.

Expression and role of gap junction protein connexin43 in immune challenge-induced extracellular ATP release in Japanese flounder (Paralichthys olivaceus)

Connexin43 (Cx43) is the best characterized gap junction protein that allows the direct exchange of signaling molecules during cell-to-cell communications. The immunological functions and ATP permeable properties of Cx43 have been insensitively examined in mammals. The similar biological significance of Cx43 in lower vertebrates, however, is not yet understood. In the present study we identified and characterized a Cx43 ortholog (termed PoCx43) from Japanese flounder (Paralichthys olivaceus) and investigated its role in immune challenge-induced extracellular ATP release. PoCx43 mRNA transcripts are widely distributed in all tested normal tissues and cells with predominant expression in the brain, and are significantly up-regulated by LPS, poly(I:C) and zymosan challenges and Edwardsiella tarda infections as well, suggesting that PoCx43 expression was modulated by the inflammatory stresses. In addition, cyclic AMP (cAMP), an essential second messenger, also plays an important role in regulating PoCx43 gene expression, by which the PoCx43-mediated gap junctional communication may be regulated. Furthermore, overexpression of PoCx43 in Japanese flounder FG-9307 cells significantly potentiates the LPS- and poly(I:C)-induced extracellular ATP release and this enhanced ATP release was attenuated by pre-incubation with Cx43 inhibitor carbenoxolone. In a complementary experiment, down-regulation of PoCx43 endogenous expression in FG-9307 cells with small interfering RNA also significantly reduced the PAMP-induced extracellular ATP release, suggesting that PoCx43 is an important ATP release conduit under the immune challenge conditions. Finally, we showed that extracellular ATP stimulation led to an increased PoCx43 expression which probably provides a feedback mechanism in regulating PoCx43 expression at the transcriptional level. These findings suggest that PoCx43 is an inducible immune response gene and an important conduit for immune challenge-induced extracellular ATP release in fish.