ATP release channel Pannexin1 is a novel immune response gene in Japanese flounder Paralichthys olivaceus

Characterization of Pannexin1, Connexin32, and Connexin43 in Spotted Sea Bass (Lateolabrax maculatus): They Are Important Neuro-Related Immune Response Genes Involved in Inflammation-Induced ATP Release

Many immunological diseases can be treated by regulating neurobehavior, in which extracellular ATP is a vital member of endogenous danger-associated molecular pattern signaling molecule that plays a crucial part in innate neuro-related immunity. It is actively released through pannexin (Panx) and connexin (Cx) hemichannels from activated or stressed cells during inflammation, injury, or apoptosis. In addition to participating in ATP release, Panxs and Cxs also have crucial immune functions. In this study, pannexin1, three connexin32 isoforms and connexin43 were identified and characterized in spotted sea bass (Lateolabrax maculatus), which were named LmPanx1, LmCx32.2, LmCx32.3, LmCx32.7, and LmCx43. Their similar topological structures were discovered by sequence analysis: a relatively unconserved C-terminal region and four highly conserved transmembrane (TM) domains, and so on. Each extracellular (ECL) region of Panx1 has two conserved cysteine residues. Unlike Panx1, each ECL region of Cx32 and Cx43 contains three conserved cysteine residues, forming two conserved motifs: CX6CX3C motif in ECL1 and CX4CX5C motif in ECL2. Furthermore, Panx1 and Cx43 share similar genomic organization and synteny with their counterparts in selected vertebrates. Cx32 and CX43 were located in the same locus in fish, but diverged into two loci from amphibian. Moreover, despite varying expression levels, the identified genes were constitutively expressed in all examined tissues. All genes were upregulated by PAMP [lipopolysaccharide and poly(I:C)] stimulation or bacterial infection in vivo and in vitro, but they were downregulated in the brain at 6 or 12 h after stimulation. Especially, the three LmCx32 isoforms and LmCx43 were upregulated by ATP stimulation in primary head kidney leukocytes; however, downregulation of LmCx32.3 and LmCx43 expression were noted at 12 h. Conversely, ATP treatment inhibited the expression of LmPanx1. Importantly, we showed that the spotted sea bass Panx1, Cx43, and Cx32 were localized on the cellular membrane and involved in inflammation-induced ATP release. Taken together, our results demonstrated that Panx1, Cx32, and Cx43 are important neuro-related immune response genes involved in inflammation-induced ATP release.

Genome-wide characterization of gap junction (connexins and pannexins) genes in turbot (Scophthalmus maximus L.): evolution and immune response following Vibrio anguillarum infection

Gap junction (GJ), a special intercellular junction between different cell types, directly connects the cytoplasm of adjacent cells, allows various molecules, ions and electrical impulses to pass through the intercellular regulatory gate, and plays vital roles in response to bacterial infection. Up to date, the information about the GJ in turbot (Scophthalmus maximus L.) is still limited. In current study, 43 gap junction genes were identified in turbot, phylogeny analysis suggested that gap junctions from turbot and other species were clustered into six groups, GJA, GJB, GJC, GJD, GJE and PANX, and turbot GJs together with respective GJs from Japanese flounder, half-smooth tongue sole and large yellow croaker, sharing same ancestors. In addition, these 43 GJ genes distributed in different chromosomes unevenly. According to gene structure and domain analysis, these genes (in GJA-GJE group) were highly conserved in that most of them contain the transmembrane area, connexin domain (CNX) and cysteine-rich domain (connexin CCC), while PANXs contain Pfam Innexin. Although only one tandem duplication was identified in turbot gap junction gene, 235 pairs of segmental duplications were identified in the turbot genome. To further investigate their evolutionary relationships, Ka/Ks was calculated, and results showed that most ratios were lower than 1, indicating they had undergone negative selection. Finally, expression analysis showed that gap junction genes were widely distributed in turbot tissues and significantly regulated after Vibrio anguillarum infection. Taken together, our research could provide valuable information for further exploration of the function of gap junction genes in teleost.

Characterization of three connexin32 genes and their role in inflammation-induced ATP release in the Japanese flounder Paralichthys olivaceus

Extracellular ATP (eATP) is a potent singling molecule in activation of fish innate immunity while the molecular determinants for eATP release in fish were not completely understood. Connexin32 (Cx32) is a member of gap junction protein family that plays important immunological functions in mammals. However, the immune relevance of Cx32 and its role in ATP release in fish has not been investigated. Here, we identified, characterized three Cx32 isoform genes (Cx32.2, Cx32.2x and Cx32.7) from the Japanese flounder Paralichthys olivaceus, and investigated their role in inflammation-induced ATP release in fish. Expression analysis revealed that even though all the three Cx32 genes are constitutively expressed in all examined Japanese flounder tissues, Cx32.2 and Cx32.2x are dominantly expressed in liver, and Cx32.7 is highly expressed in intestine and head kidney macrophages. In addition, we showed that gene expression of all the three Cx32 isoforms was modulated by cAMP stimulation and inflammatory challenges. Furthermore, we revealed that Cx32 expression was upregulated in TNF-alpha overexpressed Japanese flounder FG-9307 cells. Moreover, overexpression of the three Cx32 isoforms significantly reduced the gene expression level of LPS-induced pro-inflammatory cytokine IL-8 and TNF-alpha, indicating that Cx32 is involved in modulating inflammatory response in fish. Finally, we showed that inflammation-induced ATP release was significantly increased in Cx32-overexpressed Japanese flounder FG-9307 cells, and this increased ATP release could be attenuated by pre-incubation with gap junction protein blocker carbenoxolone. Taken together, we for the first time reported the involvement of Cx32 in fish immunity. Our findings suggested that in addition to Cx43 and pannexin1 channels, Cx32 also plays a role in inflammation-induced ATP release in fish.

Functional characterization of two ecto-nucleoside triphosphate diphosphohydrolase 2 genes in Japanese flounder (Paralichthys olivaceus) head kidney macrophages

Ecto-nucleoside triphosphate diphosphohydrolases (ENTPDases) are pivotal regulators of extracellular ATP-mediated purinergic immune signaling. ENTPDase2 is a member of the cell surface-bound ecto-nucleoside triphosphate diphosphohydrolase (ENTPDase) protein family that hydrolyzes extracellular nucleoside 5'-triphosphates and nucleoside 5'-diphosphates. However, the immune relevance of ENTPDase2 in fish has not been elucidated. In the present study, from a comparative immunological perspective, we functionally characterized two ENTPDase2 transcript variants (namely ENTPDase2 and ENTPDase2a) from Japanese flounder (Paralichthys olivaceus). Sequence analysis indicates that the deduced Japanese flounder ENTPDase2 and ENTPDase2a proteins possess two conserved transmembrane domains and five apyrase conserved regions that are present in ENTPDase family proteins. However, these proteins only share 54% amino acid sequence identity. Tissue expression analysis revealed that both ENTPDase2 and ENTPDase2a mRNA transcripts are ubiquitously expressed in all examined Japanese flounder tissues, whereas ENTPDase2 is dominantly expressed in blood and ENTPDase2a is abundantly expressed in muscle. Immune challenge experiments showed that ENTPDase2 and ENTPDase2a were significantly upregulated by both inflammatory stimulation and Edwardsiella tarda infection. In addition, the expression of ENTPDase2 and ENTPDase2a was modulated by extracellular ATP (eATP) stimulation in a dose-dependent manner. Furthermore, immunolocalization and functional studies demonstrated that both ENTPDase2 and ENTPDase2a are functional glycosylated plasma membrane proteins. However, ENTPDase2a exhibits greater activity in the hydrolysis of eATP than ENTPDase2 and ENTPDase1 proteins. Finally, knockdown of the ENTPDase2 gene by small interfering RNA significantly upregulated the expression of eATP-induced proinflammatory cytokines IL-1beta, TNF-alpha and G-CSF in Japanese flounder head kidney macrophages, while knockdown of ENTPDase2a only upregulated eATP-induced IL-1beta expression. Taken together, our findings suggest that the two functional Japanese flounder ENTPDase2 isoforms play an essential role in the downregulation of eATP-induced proinflammatory cytokine expression in fish by degrading the available ATP levels in the extracellular milieu.

Identification and characterization of apoptosis-related gene serine/threonine kinase 17A (STK17A) from Japanese flounder Paralichthys olivaceus

Apoptosis plays important roles in regulation of the immune response and has a direct impact on disease resistance in teleost. Death associated protein kinase (DAPK)-related Serine/Threonine kinase 17A (STK17A) is a positive apoptosis regulator. However, the expression and function of STK17A in fish still remains uninvestigated. In this study, we identified and characterized a STK17A gene (termed PoSTK17A) from Japanese flounder Paralichthys olivaceus. We also investigated the pro-apoptotic role of PoSTK17A in fish. Real-time quantitative PCR analysis revealed that PoSTK17A is widely present in various Japanese flounder tissues, and dominantly expressed in liver. Immune challenge experiments showed that PoSTK17A expression was upregulated by inflammatory challenge, Edwardsiella tarda infection and DNA-damaging agent cisplatin treatment as well. Immunofluorescence microscopy revealed that the recombinant PoSTK17A proteins are mainly located in the nucleus of Japanese flounder FG-9307 cells, and human Hela and MCF7 cells. However, PoSTK17A was translocated from the nucleus to cytoplasm following cisplatin treatment. Overexpression of PoSTK17A significantly increased the apoptosis in human MCF7 cells through both cisplatin-dependent and independent manners. Importantly, PoSTK17A also promotes the ATP-gated P2X7 receptor-mediated apoptosis in Japanese flounder FG-9307 cells. Collectively, we characterized an inducible STK17A gene (PoSTK17A) that may play a conserved pro-apoptotic role in fish.

Purinergic signaling and gene expression of purinoceptors in the head kidney of the silver catfish Rhamdia quelen experimentally infected by Flavobacterium columnare

The head kidney is a lymphoid immune organ that plays a key role in the immune and inflammatory responses of teleost fish. It is associated with immunoglobulin G production and differentiation of B cells. The presence of a multi-enzymatic complex found anchored in the plasma membrane makes the head kidney an important purinergic-dependent tissue. Purinergic signaling has been associated with these responses under pathological conditions via regulation of extracellular adenosine triphosphate (ATP), the main damage molecular associated pattern agent released during bacterial infections. The aim of this study was to determine whether purinergic signaling is a pathway associated with impairment of immune responses in silver catfish (Rhamdia quelen) experimentally infected by Flavobacterium columnare, as well as to evaluate the role of P2 purine receptors in this response. Triphosphate diphosphohydrolase (NTPDase) activity in the head kidney was significantly lower in silver catfish experimentally-infected F. columnare 72 h post-infection (hpi) than in the control group, while no significant difference was observed with respect NTPDase activity on adenosine diphosphate, as well as on 5'-nucleotidase and adenosine deaminase activities. Extracellular ATP levels were significantly higher in the head kidney of experimentally-infected fish than in the control group at 72 hpi. Finally, p2ry11 and p2rx3 purine receptor levels were significantly higher in experimentally-infected fish than in the control group at 72 hpi. We conclude that purinergic signaling in the head kidney of silver catfish infected by F. columnare creates a pro-inflammatory profile that may contribute to impairment of immune and inflammatory responses via reduction of ATP hydrolysis and its accumulation in the extracellular milieu, accompanied by upregulation of p2ry11 and p2rx3 purine receptors, leading to pro-inflammatory status.

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 the responses of the caspase 2, 3, 6 and 8 genes to immune challenges and extracellular ATP stimulation in the Japanese flounder (Paralichthys olivaceus)

BACKGROUND

Caspases are a family of conserved intracellular cysteine-dependent aspartate-specific cysteine proteases that play important roles in regulating cell death and inflammation. Our previous study revealed the importance of the inflammatory caspase 1 gene in extracellular ATP-mediated immune signaling in Japanese flounder, Paralichthys olivaceus. To explore the potential roles of other caspases in P. olivaceus innate immunity, we extended our study by characterizing of the responses of four additional P. olivaceus caspase genes, termed JfCaspase 2, 3, 6 and 8, to inflammatory challenge and extracellular ATP stimulation.

RESULTS

Sequence analysis revealed that the domain structures of all the Japanese flounder caspase proteins are evolutionarily conserved. Quantitative real-time PCR analysis showed that the JfCaspase 2, 3, 6 and 8 genes were expressed ubiquitously but at unequal levels in all examined Japanese flounder normal tissues. In addition, the basal gene expression levels of JfCaspase 2, 3, 6 and 8 were higher than those of JfCaspase 1 in both Japanese flounder head kidney macrophages (HKMs) and peripheral blood leukocytes (PBLs). Furthermore, immune challenge experiments showed that the inflammatory stimuli LPS and poly(I:C) significantly modulated the expression of the JfCaspase 2, 3, 6 and 8 genes in Japanese flounder immune cells. Finally, DNA fragmentation, associated with increased extracellular ATP-induced JfCaspase 2, 3, 6 and 8 gene expression and enzymatic activity, was inhibited by the caspase inhibitor Z-VAD-FMK in the HKMs.

CONCLUSION

Our findings demonstrate broad participation of multiple caspase genes in response to inflammatory stimulation in Japanese flounder immune cells and provide new evidence for the involvement of caspase(s) in extracellular ATP-induced apoptosis in fish.

Expression analysis of Pannexin1 channel gene in response to immune challenges and its role in infection-induced ATP release in tilapia (Oreochromis niloticus)

ATP released from immune cells plays an important role in activation of host innate immunity. However, the molecular mechanisms for pathogen infection-induced ATP release in fish remains unclear. Pannexin1 (Panx1) is a recently identified ATP release channel important for controlling immune responses. The immune relevance of Panx1 in fish, however, is still poorly understood. In this study, we characterized a Panx1 gene homologue (termed tPanx1) from Nile tilapia (Oreochromis niloticus) and analyzed its expression in response to different immune challenges. We also investigated the role of tPanx1 channel in bacterial infection-induced ATP release. Real-time quantitative PCR analysis revealed that tPanx1 gene is expressed in all tested tissues with predominant expression in intestine. Immune challenges with lipopolysaccharide, polyinosinic-polycytidylic acid and zymosan led to increased gene expression of tPanx1 in tilapia head kidney cells and peripheral blood leucocytes. In addition, tPanx1 gene was up-regulated in hepatopancreas, muscle, spleen, gill, head kidney and blood after Aeromonas hydrophila infection. Furthermore, pharmacological inhibition of tPanx1 channel activity with Panx1 channel inhibitor, carbenoxolone, significantly attenuated A. hydrophila infection-induced ATP release in tilapia head kidney cells. Taken together, our findings suggested that tPanx1 is an important immune response gene involved in bacterial infection-induced ATP release in tilapia O. niloticus.

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.

Interactions of Pannexin1 channels with purinergic and NMDA receptor channels

Pannexins are a three-member family of vertebrate plasma membrane spanning molecules that have homology to the invertebrate gap junction forming proteins, the innexins. However, pannexins do not form gap junctions but operate as plasma membrane channels. The best-characterized member of these proteins, Pannexin1 (Panx1) was suggested to be functionally associated with purinergic P2X and N-methyl-D-aspartate (NMDA) receptor channels. Activation of these receptor channels by their endogenous ligands leads to cross-activation of Panx1 channels. This in turn potentiates P2X and NMDA receptor channel signaling. Two potentiation concepts have been suggested: enhancement of the current responses and/or sustained receptor channel activation by ATP released through Panx1 pore and adenosine generated by ectonucleotidase-dependent dephosphorylation of ATP. Here we summarize the current knowledge and hypotheses about interactions of Panx1 channels with P2X and NMDA receptor channels. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Comparative analysis of dual specificity protein phosphatase genes 1, 2 and 5 in response to immune challenges in Japanese flounder Paralichthys olivaceus

Dual-specificity MAP kinase (MAPK) phosphatases (DUSPs) are well-established negative modulators in regulating MAPK signaling in mammalian cells and tissues. Our previous studies have shown the involvement of DUSP6 in regulating innate immunity in Japanese flounder Paralichthys olivaceus. In order to gain a better understanding of the role of DUSPs in fish innate immunity, in the present study we identified and characterized three additional DUSP genes including DUSP1, 2 and 5 in P. olivaceus. The three Japanese flounder DUSP proteins share common domain structures composed of a conserved N-terminal Rhodanase/CDC25 domain and a C-terminal catalytic phosphatase domain, while they show only less than 26% sequence identities, indicating that they may have different substrate selectivity. In addition, mRNA transcripts of all the three DUSP genes are detected in all examined Japanese flounder tissues; however, DUSP1 is dominantly expressed in spleen while DUSP2 and 5 are primarily expressed in skin. Furthermore, all the three DUSP genes are constitutively expressed in the Japanese flounder head kidney macrophages (HKMs) and peripheral blood leucocytes (PBLs) with unequal distribution patterns. Moreover, all the three DUSPs gene expression was induced differently in response to the LPS and double-stranded RNA mimic poly(I:C) stimulations both in the Japanese flounder HKMs and PBLs, suggesting an association of DUSPs with TLR signaling in fish. Taken together, the co-expression of various DUSPs members together with their different responses to the immune challenges indicate that the DUSP members may operate coordinately in regulating the MAPK-dependent immune responses in the Japanese flounder.

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.

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

P2X2 receptor (P2X2R) belongs to the family of purinergic receptors that have been shown to play important roles in regulating host innate immune response. Although the immunologic significance of P2X2R has been studied in mammals, the presence and immune relevance of P2X2R in fish remains unclear. In this study we extended our previous observations by identifying and characterizing a P2X2R ortholog (termed PoP2X2R) from Japanese flounder (Paralichthys olivaceus). Quantitative real-time PCR analysis revealed that PoP2X2R mRNA transcripts are widely distributed in all examined normal tissues and are dominantly expressed in hepatopancreas tissue. In addition, we for the first time showed that multiple P2XR subtypes, including P2X2R, P2X4R and P2X7R are co-expressed in the Japanese flounder head kidney macrophages (HKMs) and peripheral blood lymphocytes (PBLs), indicating that they may assemble into hetero-receptor complex or interact in the form of homotrimers to trigger diverse purinergic signaling in the Japanese flounder immune cells. Compared with the known Japanese flounder P2X4 and P2X7 receptors, however, PoP2X2R is much more abundantly expressed in the Japanese flounder HKM cells, suggesting that PoP2X2R may play an important role in this type of immune cells. Glycosylation and immunohistochemistry analyses revealed that PoP2X2R is a glycoprotein expressed on the plasma membrane. Immune challenges experiments showed that PoP2X2R was significantly induced by LPS, poly(I:C) and zymosan stimulations in the HKM and PBL cells, and by Edwardsiella tarda infections in spleen and gill tissues as well. Taken together, we have identified and characterized a new P2X2R member that is involved in fish innate immune response.

Identification and characterization of a novel NOD-like receptor family CARD domain containing 3 gene in response to extracellular ATP stimulation and its role in regulating LPS-induced innate immune response in Japanese flounder (Paralichthys olivaceus) head kidney macrophages

Nucleotide oligomerization domain (NOD)-like receptor (NLR) family with a caspase activation and recruitment domain (CARD) containing 3 (NLRC3) protein is an important cytosolic pattern recognition receptor that negatively regulates innate immune response in mammals. Hitherto, the immunological significance of NLRC3 protein in fish remains largely uncharacterized. Here we identified and characterized a novel NLRC3 gene (named poNLRC3) implicated in regulation of fish innate immunity from Japanese flounder Paralichthys olivaceus. The poNLRC3 protein is a cytoplasmic protein with an undefined N-terminal domain, a NACHT domain, a fish-specific NACHT associated domain, six LRR motifs, and a C-terminal fish-specific PYR/SPYR (B30.2) domain but only shares less than 40% sequence identities with the known Japanese flounder NLRC proteins. poNLRC3 gene is ubiquitously expressed in all tested tissues and is dominantly expressed in the Japanese flounder head kidney macrophages (HKMs). We for the first time showed that poNLRC3 expression was significantly modulated by the stimulation of extracellular ATP, an important danger/damage-associated molecular pattern in activating innate immunity in P. olivaceus. Importantly, we revealed that poNLRC3 plays an important role in positively regulating ATP-induced IL-1beta and IL-6 gene expression, suggesting the involvement of poNLRC3 in extracellular ATP-mediated immune signaling. In addition, we showed that poNLRC3 mRNA expression was up-regulated in response to LPS and Edwardsiella tarda immune challenges. Finally, we showed that down-regulating the endogenous poNLRC3 expression with small interfering RNA significantly reduced LPS-induced proinflammatory cytokine gene expression in the Japanese flounder HKM cells. Altogether, we have identified a novel inducible fish NLR member, poNLRC3, which is involved in extracellular ATP-mediated immune signaling and may positively regulate the LPS-induced innate immune response in the Japanese flounder HKM cells.

A capillary zone electrophoresis method for adenine nucleotides analysis in Saccharomyces cerevisiae

Adenosine triphosphate and its metabolites are involved in the cellular metabolism process in Saccharomyces cerevisiae. It is very important to simultaneously determine the relative contents of ATP and its metabolites in yeast. In this study, an effective capillary zone electrophoresis method with high selectivity was established. The calibration curves were linear in the concentration range from 1 to 20mg/L (ATP and cAMP) and 2 to 40mg/L (ADP and AMP) with excellent correlation coefficients (r(2))>0.999. The recovery of ATP, ADP, AMP, and cAMP were 99.4%, 94.7%, 100.3% and 99.6%, respectively. Simple sample preparation and easy detection of ATP and its metabolites make this method suitable for the study of changes in the four adenine nucleotides levels caused by caloric restriction in yeast. It is expected that the current method may contribute to further energy metabolism and related investigations of yeast.

A novel innexin2 forming membrane hemichannel exhibits immune responses and cell apoptosis in Scylla paramamosain

Innexins are a class of transmembrane proteins that are important for embryonic development, morphogenesis and electrical synapse formation. In the present study, a novel innexin2 gene from Scylla paramamosain was named Sp-inx2 and characterized. The complete cDNA and genomic DNA sequences of Sp-inx2 were revealed. Sp-inx2 mRNA transcripts were distributed in various tissues of S. paramamosain and were most abundant in the hemocytes. The Sp-inx2 was significantly upregulated in hemocyte, gill and hepatopancreas tissues with the challenge of either Vibrio alginolyticus, Vibrio parahaemolyticus or lipopolysaccharides (LPSs) when analyzed at 3 and 6 h using quantitative real-time PCR, suggesting that it could activate an immune response against the challenge of LPSs or Vibrio species. Using the chemical inhibitors carbenoxolone and probenecid, the absorption of the fluorescent dye Lucifer yellow decreased in the primary cultured hemocytes of crabs, thus confirming that hemichannels composed of Sp-inx2 existed in the crab hemocytes. With LPS stimulation, the level of mRNA transcripts and protein expression of Sp-inx2 in the same cultured hemocytes gradually increased from 6 to 48 h, while the activity of hemichannels was down-regulated at 6 and 12 h, demonstrating that LPSs could modulate the absorption activity of hemichannels in addition to its upregulation of Sp-inx2 gene expression. Furthermore, the dye uptake rate in HeLa cells in which Sp-inx2 was ectopically expressed increased dramatically but the increase was significantly down-regulated with the addition of 50 μg mL(-1) LPS, suggesting that the LPS stimulation could effectively reduce the activity of hemichannels. Interestingly, with the ectopic expression of Sp-inx2 in HeLa and EPC cells, apoptosis spontaneously occurred in both cultured cell lines when detected using TUNEL assay. In summary, a new Sp-inx2 gene was first characterized in a marine animal S. paramamosain and it had a function associated with immune response and cell apoptosis.

Molecular characterization of purinergic receptor P2X4 involved in Japanese flounder (Paralichthys olivaceus) innate immune response and its interaction with ATP release channel Pannexin1

P2X4 receptor (P2X4R) is a member of trimeric ATP-gated receptor channel family. Despite the importance of P2X4R in innate immunity has been addressed in mammals, the immunological significance of P2X4R has not been characterized in fish. In the present study we identified a full-length P2X4R cDNA sequence from Japanese flounder Paralichthys olivaceus (termed poP2X4R) by RT-PCR and RACE approaches and analyzed its gene expression patterns under normal and immune challenge conditions. Qualitative RT-PCR analyses revealed that poP2X4R has a widespread distribution in all examined tissues but dominantly expressed in hepatopancreas. In Japanese flounder head kidney macrophages and peripheral blood lymphocytes, poP2X4R was rapidly and significantly up-regulated by the immune challenges of LPS, poly(I:C) and zymosan. In addition, poP2X4R was up-regulated in spleen, head kidney and gill tissues by Edwardsiella tarda infections. Furthermore, we showed that poP2X4R is a membrane glycoprotein which could interact with ATP release channel Pannexin1, an important component in extracellular ATP-activated purinergic signaling pathways involved in Japanese flounder innate immune response. From a comparative immunological point of view, our results have provided new evidence for the involvement of extracellular ATP-gated P2XRs in fish innate immunity.

Identification and expression analysis of nascent polypeptide-associated complex alpha gene in response to immune challenges in Japanese flounder Paralichthys olivaceus

Nascent polypeptide-associated complex (NAC) is a conserved heterodimeric protein consisting of alpha and beta subunits. In addition to acting as a protein translation chaperone by forming a heterodimer with the beta subunit, NAC alpha (NACA) also shows important immune significance independent of NAC beta in mammalian cells. In lower vertebrates, however, the immunological relevance of NACA has not been revealed yet. In the present study, we identified and characterized a NACA gene (termed poNACA) involved in innate immune response in Japanese flounder Paralichthys olivaceus. poNACA encodes a 215-amino-acid protein, with an apparent molecular weight of 23.5 kDa and an isoelectric point of 4.51. Tissue distribution analysis revealed that poNACA gene was constitutively expressed in all examined tissues and showed dominant expression in hepatopancreas and gonad tissues. In enriched Japanese flounder head kidney macrophages and peripheral blood leucocytes, the expression of poNACA mRNA transcript was significantly induced by LPS, Poly(I:C) and zymosan stimulations. In vivo experiments further revealed that poNACA gene expression was up-regulated in head kidney, gill and spleen tissues in response to Edwardsiella tarda challenges. Furthermore, overexpression of poNACA in Japanese flounder FG-9307 cells resulted in increased gene expression of IL-1beta, IL-11 and TNF-alpha, and myxovirus resistance (Mx). Taken together, our findings indicate that an immune response gene, poNACA, involved in innate immune regulation in P. olivaceus has been identified.