Functional characterization of a short peptidoglycan recognition protein, PGRP5 in grass carp Ctenopharyngodon idella

LGP2 Facilitates Bacterial Escape through Binding Peptidoglycan via EEK Motif and Suppressing NOD2-RIP2 Axis in Cyprinidae and Xenocyprididae Families

RIG-I-like receptors and NOD-like receptors play pivotal roles in recognizing microbe-associated molecular patterns and initiating immune responses. The LGP2 and NOD2 proteins are important members of the RIG-I-like receptor and NOD-like receptor families, recognizing viral RNA and bacterial peptidoglycan (PGN), respectively. However, in some instances bacterial infections can induce LPG2 expression via a mechanism that remains largely unknown. In the current study, we found that LGP2 can compete with NOD2 for PGN binding and inhibit antibacterial immunity by suppressing the NOD2-RIP2 axis. Recombinant CiLGP2 (Ctenopharyngodon idella LGP2) produced using either prokaryotic or eukaryotic expression platform can bind PGN and bacteria in pull-down and ELISA assays. Comparative protein structure models and intermolecular interaction prediction calculations as well as pull-down and colocalization experiments indicated that CiLGP2 binds PGN via its EEK motif with species and structural specificity. EEK deletion abolished PGN binding of CiLGP2, but insertion of the CiLGP2 EEK motif into zebrafish and mouse LGP2 did not confer PGN binding activity. CiLGP2 also facilitates bacterial replication by interacting with CiNOD2 to suppress expression of NOD2-RIP2 pathway genes. Sequence analysis and experimental verification demonstrated that LGP2 having EEK motif that can negatively regulate antibacterial immune function is present in Cyprinidae and Xenocyprididae families. These results show that LGP2 containing EEK motif competes with NOD2 for PGN binding and suppresses antibacterial immunity by inhibiting the NOD2-RIP2 axis, indicating that LGP2 plays a crucial negative role in antibacterial response beyond its classical regulatory function in antiviral immunity.

Short peptidoglycan recognition protein 5 modulates immune response to bacteria in Indian major carp, Cirrhinusmrigala

Peptidoglycan recognition proteins (PGRPs) function in host antibacterial responses by recognizing bacterial peptidoglycan (PGN). In the present study, a short pgrp5 (named mpgrp5) was identified in Cirrhinus mrigala (mrigal). The full-length cDNA of the mpgrp5 gene was 1255 bp, containing an open reading frame of 746 bp encoding a protein of 248 amino acids. The predicted protein contained the typical Pgrp/amidase domain, conserved Zn2+, and PGN binding residues. The phylogenetic analysis revealed that the mpgrp5 is closely related to Pgrps reported in Labeo rohita, Cyrinus carpio, and Ctenopharyngodon idella. The ontogenetic expression of mpgrp5 was highest at 7 days post-hatching (dph) and its possible maternal transfer. mpgrp5 was constitutively expressed in all tissues examined, with the highest expression observed in the intestine. Furthermore, mpgrp5 was found upregulated in mrigal post-challenge in a time-dependent manner at 6hpi in the liver (3.16 folds, p < 0.05) and kidney (2.79 folds, p < 0.05) and at 12hpi in gill (1.90 folds, p < 0.01), skin (1.93 folds, p < 0.01), and intestine, (2.71 folds, p < 0.05) whereas at 24hpi in spleen (4.0 folds, p < 0.01). Our results suggest that mpgrp5 may play an important role in antibacterial immune response from early life stages in mrigal.

Identification and characterization of two long-type peptidoglycan recognition proteins, PGRP-L1 and PGRP-L2, in the orange-spotted grouper, Epinephelus coioides

Peptidoglycan recognition proteins (PGRPs) play an important role in innate immunity by recognizing components of pathogenic bacteria (such as peptidoglycan, PGN) and are evolutionarily conserved pattern recognition receptors (PRRs) in both invertebrates and vertebrates. In the present study, two long-type PGRPs (designed as Eco-PGRP-L1 and Eco-PGRP-L2) were identified in orange-spotted grouper (Epinephelus coioides), which is a major economic species cultured in Asia. The predicted protein sequences of both Eco-PGRP-L1 and Eco-PGRP-L2 contain a typical PGRP domain. Eco-PGRP-L1 and Eco-PGRP-L2 exhibited organ/tissue-specific expression patterns. An abundant expression of Eco-PGRP-L1 was observed in pyloric caecum, stomach and gill, whereas a highest expression level of Eco-PGRP-L2 was found in head kidney, spleen, skin and heart. In addition, Eco-PGRP-L1 is distributed in the cytoplasm and nucleus, while Eco-PGRP-L2 is mainly localized in cytoplasm. Both Eco-PGRP-L1 and Eco-PGRP-L2 were induced following the stimulation of PGN and have PGN binding activity. In addition, functional analysis revealed that Eco-PGRP-L1 and Eco-PGRP-L2 possess antibacterial activity against Edwardsiella tarda. These results may contribute to understand the innate immune system of orange-spotted grouper.

Peptidoglycan recognition protein MsPGRP in largemouth bass (Micropterus salmoides) mediates immune functions with broad nonself recognition ability

Peptidoglycan (PGN) recognition proteins (PGRPs) are important immune factors in innate immunity that function in recognising pathogens and activating the immune system. These ubiquitous proteins are conserved in invertebrates and vertebrates. In this study, a PGRP gene (MsPGRP) from largemouth bass (Micropterus salmoides) was identified and characterised, and its transcription distribution was explored. Recombinant protein (rMsPGRP) exhibited dose-dependent binding to PGN and glucan (GLU), but weak binding to lipopolysaccharide (LPS). MsPGRP exhibited agglutinating activity against several Gram-negative bacteria, Gram-positive bacteria and fungi, and it promoted phagocytosis activity of leukocytes against Micrococcus luteus and Aeromonas hydrophila. The protein also possessed amidase activity in the presence of Zn²⁺, degraded PGN, and disrupted the M. luteus cell wall. The results suggest that MsPGRP plays an important role in pathogen recognition, and acts as a opsonin during immune system responses and elimination of invading pathogens.

Common carp Peptidoglycan Recognition Protein 2 (CcPGRP2) plays a role in innate immunity for defense against bacterial infections

PGRP is a family of pattern recognition molecules of the innate immune system. PGRPs are conserved from insects to mammals and have diverse functions in antimicrobial defense. Here we cloned a common carp PGRP ortholog, CcPGRP2 containing a conserved C-terminal PGRP domain. We tested the expression levels of CcPGRP2 in the liver, spleen, kidney, foregut, midgut, and hindgut of the highest level in the liver. The expression of CcPGRP2 upregulated in common carp infected with Aeromonas hydrophila (A. hydrophila) or Staphylococcus aureus (S. aureus). Recombinant CcPGRP2 protein expressed in Escherichia coli (E. coli) system and the purified CcPGRP2 could maintain the integrity of intestinal mucosa of common carp infected with A. hydrophila. In addition, CcPGRP2 could agglutinate or bind both gram-positive and gram-negative bacteria in a Zn²⁺-dependent manner. CcPGRP2 has a stronger agglutination and bacterial binding ability in gram-positive bacteria than in gram-negative bacteria. It is perhaps because CcPGRP2 could bind peptidoglycan (PGN) with a higher degree to lipopolysaccharide (LPS). And CcPGRP2 shows antimicrobial activities in the presence of Zn²⁺. Our results of CcPGRP2 provided new insight into the function of PGRP in the innate immunity of the common carp.

Identification of PGRP2 and its three splice variants in grass carp Ctenopharyngodon idella

In this study, peptidoglycan recognition protein 2 (PGRP2) gene was cloned in grass carp Ctenopharyngodon idella, with the open reading frame (ORF) of PGRP2 being 1452 bp, encoding a protein of 483 amino acids. Three splice variants, PGRP2a, PGRP2b, and PGRP2c, were found also in grass carp with the absence of entire exon two and partial exon two of the PGRP2, and were predicted to have 124, 371 and 311 amino acids. But, they all have PGRP domain and signal peptide, except PGRP2a. The PGRP2 and its variants were expressed in all organs/tissues examined, and stimulated following PGN injection. It is further detected that the expression of gcPGRP2 and its variants was up-regulated after the single transfection of each of gcPGRP2 and its variant expression plasmids in CO cells. It is considered that the cloning of PGRP2 in grass carp provides a compositional completeness of PGRP members in this fish with the inclusion of previously reported PGRP5 and PGRP6.

Peptidoglycan-based immunomodulation

Peptidoglycan (PGN) is a unique component in the cytoderm of prokaryotes which can be recognized by different pathogen-associated molecular patterns (PAMPs) in eukaryotes, followed by a cascade of immune responses via different pathways. This review outlined the basic structure of PGN, its immunologic functions. The immunomodulation pathways mediated by PGN were elaborated. PGN induces specific immunity through stimulating different cytokine release and Th1/Th2-dominated immune responses during humoral/cellular immune response. The nonspecific immunity activation by PGN involves immunomodulation by different pattern recognition receptors (PRRs) including PGN recognition proteins (PGRPs), nucleotide oligomerization domain (NOD)-like receptors (NLRs), Toll-like receptors (TLRs), and C-type lectin receptors (CLRs). The sources and classification of PGRPs were summarized. In view of the stimulating activities of PGN and its monomers, the potential application of PGN as vaccine or adjuvant was prospected. This review provides systematic information on PGN functionalities from the point of immunoregulation, which might be useful in the deep exploitation of PGN.Key points. The immunological functions of PGN were illustrated. Cellular and humoral immunomodulation by PGN were outlined. The use of PGN as vaccine or adjuvant was prospected.

Molecular identification of peptidoglycan recognition protein 5 and its functional characterization in innate immunity of large yellow croaker, Larimichthys crocea

Fish peptidoglycan recognition proteins (PGRPs) play important roles in microbial recognition, and bacterial elimination. In the present study, a short-type PGRP from large yellow croaker, LcPGRP5 was cloned and its functions were characterized. LcPGRP5 gene encodes a protein containing conserved PGRP domain, but no signal peptide. Phylogenetic analysis shows that LcPGRP5 is clustered with other short PGRPs identified in other teleosts. LcPGRP5 is constitutively expressed in all tissues examined, with the highest expression being detected in the head kidney. Recombinant LcPGRP5 protein features amidase activity and bactericidal activity. Notably, LcPGRP5 could enhance the phagocytosis of the bacteria by large yellow croaker macrophage, with higher phagocytic capacity being observed in Staphylococcus aureus compared to Escherichia coli. Moreover, overexpression of LcPGRP5 suppresses pro-inflammatory effects elicited by bacterial exposure in the macrophage cell line. Overall, the present results clearly indicate the important roles of LcPGRP5 played in the innate immune responses against bacterial infection.

Gene identification and functional analysis of peptidoglycan recognition protein from the spotted sea bass (Lateolabrax maculatus)

Peptidoglycan recognition proteins (PGRPs), which are structurally conserved innate immune molecules in invertebrate and vertebrate animals, play the important roles in regulation of innate immune responses. In this paper, three PGRP genes of spotted sea bass, Lateolabrax maculatus, were cloned, designated as Ssb-PGRP2, Ssb-PGRP-L2 and Ssb-PGRP-SC2, respectively. Sequence analysis showed that the deduced amino acid sequences of Ssb-PGRP2, Ssb-PGRP-L2 and Ssb-PGRP-SC2 proteins contained respectively 468, 482 and 167 amino acid residues, and had the typical structural features of PGRPs, i.e. conserved PGRP domain and Zn²⁺ binding domain including four specific amino acid residues which were required for amidase activity. q-PCR analysis of total mRNA showed that the mRNA expression of three PGRP genes were detected in all the examined tissues and the expression patterns of Ssb-PGRP2, Ssb-PGRP-L2 and Ssb-PGRP-SC2 were different. After injected with LPS, Poly (I:C) and Edwardsiella tarda, there was a clear time-dependent expression pattern for each of the three PGRP genes in head kidney, spleen, intestine and gill of the spotted sea bass. In our study, three recombinant proteins corresponding to the three members of the peptidoglycan recognition protein family were expressed and purified. Moreover, all of the three recombinant PGRP proteins significantly inhibited bacterial survival and growth, and expressed bactericidal effects on Vibrio harveyi, Staphylococcus aureus and Edwardsiella tarda. In particular, it was firstly verified that their antimicrobial activity presented the superimposed effect. Overall, these findings indicated that three PGRP genes of spotted sea bass were at least involved in host defense against bacterial infections.

Identification of immune-related proteins of Dreissena polymorpha hemocytes and plasma involved in host-microbe interactions by differential proteomics

Biological responses of zebra mussel Dreissena polymorpha are investigated to assess the impact of contaminants on aquatic organisms and ecosystems. In addition to concentrate chemical contaminants in their tissues, zebra mussels accumulate several microorganisms such as viruses, protozoa and bacteria. In order to understand the molecular mechanisms involved in the defence against microorganisms this study aims at identifying immune proteins from D. polymorpha hemolymph involved in defence against protozoa and viruses. For this purpose, hemolymph were exposed ex vivo to Cryptosporidium parvum and RNA poly I:C. Differential proteomics on both hemocytes and plasma revealed immune proteins modulated under exposures. Different patterns of response were observed after C. parvum and RNA poly I:C exposures. The number of modulated proteins per hemolymphatic compartments suggest that C. parvum is managed in cells while RNA poly I:C is managed in plasma after 4 h exposure. BLAST annotation and GO terms enrichment analysis revealed further characteristics of immune mechanisms. Results showed that many proteins involved in the recognition and destruction of microorganisms were modulated in both exposure conditions, while proteins related to phagocytosis and apoptosis were exclusively modulated by C. parvum. This differential proteomic analysis highlights in zebra mussels modulated proteins involved in the response to microorganisms, which reflect a broad range of immune mechanisms such as recognition, internalization and destruction of microorganisms. This study paves the way for the identification of new markers of immune processes that can be used to assess the impact of both chemical and biological contaminations on the health status of aquatic organisms.

Molecular and functional characterization of a short-type peptidoglycan recognition protein, PGRP-S in the amphibian Xenopus laevis

Peptidoglycan recognition proteins (PGRPs) are a family of pattern recognition receptors (PRRs) involved in host antibacterial responses, and their functions have been characterized in most invertebrate and vertebrate animals. However, little information is available regarding the function of frog PGRPs. In this study, a short-type PGRP (termed Xl-PGRP-S) gene was identified in the African clawed frog, Xenopus laevis. The predicted protein of Xl-PGRP-S contains several structural features known in PGRPs, including a typical PGRP domain and two closely spaced conserved cysteines. Xl-PGRP-S gene was constitutively expressed in all tissues examined, with the highest expression level observed in muscle. As a typical PRR, Xl-PGRP-S is inducible after peptidoglycan (PGN) stimulation, and has an ability to bind PGN. In addition, Xl-PGRP-S has been proven to have Zn²⁺-dependent amidase activity and antibacterial activity against Edwardsiella tarda. The present study represents the first discovery on the function of frog PGRPs, thus contributing to a better understanding of the functional evolution of PGRPs in early tetrapods.

Molecular characterization and expression analysis of two peptidoglycan recognition proteins (CcPGRP5, CcPGRP6) in larvae ontogeny of common carp Cyprinus carpio L. and upon immune stimulation by bacteria

Background

Although teleost fish developed acquired immunity firstly in evolution, innate immunity is still very important for them. Innate immunity depends on pattern recognition receptors (PRRs) to distinguish “self” and “non-self”, Peptidoglycan (PGN) recognition protein (PGRP) is one of the receptors and it can bind to multiple components of bacterial envelope.

Results

We report the cloning and expression analysis of two PGRPs (Ccpgrp5 and Ccpgrp6) from common carp (Cyprinus carpio L). The Ccpgrp5 gene encodes a protein of 199 amino acid (aa) with PGRP domain, Ami_2 domain and four Zn²⁺ binding sites required for amidase activity, but without signal peptide and transmembrane domain. The Ccpgrp6 gene encodes a protein of 446 aa with PGRP domain, Ami_2 domain, signal peptide, five Zn²⁺ binding sites required for amidase activity and two sites for N-glycosylation. The phylogenetic analysis revealed that the CcPGRP5 and CcPGRP6 are closely related to Ctenopharyngodon idella and Danio rerio. Ccpgrp5 and Ccpgrp6 were expressed in all tissues examined including liver, spleen, muscle, oral epithelium, head kidney, gill, skin, gonad, brain, foregut and hindgut and showed different distribution characteristics. During the embryonic and early larval developmental stages of common carp, Ccpgrp6 was detected to be highly expressed at 10 days post fertilization(dpf) and 36 dpf, while Ccpgrp5 were hardly detected using Real-time quantitative PCR. After being challenged with Aeromonas hydrophila, Ccpgrp5 in adult common carp was induced and up-regulated in all the tissues, especially in gill and spleen, but not in head kidney, while Ccpgrp6 was up-regulated in all the tissues, especially in liver, head kidney and gill. The varied expression profiling of Ccpgrp5 and Ccpgrp6 indicated they had different roles in the host immune response.

Conclusions

These results indicated the two PGRPs, especially Ccpgrp6, played an important role in the immune defense of common carp during larva development and against Aeromonas hydrophila, providing insight to further exploration of protecting fish against bacteria infectious disease.

Involvement of a short-type peptidoglycan recognition protein (PGRP) from Chinese giant salamanders Andrias davidianus in the immune response against bacterial infection

PGRPs (Peptidoglycan recognition proteins) could recognize peptidoglycan and play vital roles in innate immunity among different animals. Till present, the functions of PGRP have been studied in various animals, but few reports have studied the amphibian PGRPs. In the current research, a short type PGRP was identified from Chinese giant salamander and its involvement in the innate immunity was studied. The ORF of AdPGRP-SC2 cDNA was 573 bp, which encoded 190 amino acids, and contained a PGRP and an amidase_2 domain. The qPCR analysis revealed that AdPGRP-SC2 mRNA transcripts expressed in different tissues, with the highest expression level in muscle, intestine and spleen. Results of immune challenges with peptidoglycan (PGN) demonstrated that expression patterns of AdPGRP-SC2 were significantly up-regulated in erythrocyte and spleen at the early injection stage. The recombinant AdPGRP-SC2 protein was successfully produced and purified, and it could show binding affinity to different bacteria. In the presence of Zn²⁺, the rAdPGRP-SC2 could exhibit a broad PAMPs binding activities, strongly agglutinate bacteria and exhibit amidase enzyme activity. Collectively, these data indicate AdPGRP-SC2 could act as PRR to recognize the invading microorganisms and as the antimicrobial effectors during the innate immune response of A. davidianus.

Two Novel Short Peptidoglycan Recognition Proteins (PGRPs) From the Deep Sea Vesicomyidae Clam Archivesica packardana: Identification, Recombinant Expression and Bioactivity

Vesicomyidae clams are common species living in cold seeps, which incorporates symbiotic bacteria into their body maintaining endosymbiosis relationship. As members of pattern recognition receptor (PRR) family, peptidoglycan recognition proteins (PGRPs) recognize pathogen associated molecular patterns and play an important role in innate immunity. In present study, two short PGRPs (ApPGRP-1 and -2) were first identified from Vesicomyidae clam Archivesica packardana. Sequences analysis showed that they have both conserved Zn²⁺ binding sites (H-H-C) and amidase catalytic sites (H-Y-H-T-C), and phylogenetic tree indicated that they clustered with short PGRPs of other molluscs. PGN assay showed that ApPGRPs could bind Lys-type PGN from Staphylococcus aureus and Dap-type PGN from Bacillus subtilis, and revealed amidase activity with selective zinc ion dependence. rApPGRP-1 and -2 (recombinant ApPGRP-1 and -2) could bind six bacteria with a broad spectrum and had both zinc-dependent and -independent bactericidal activity. ApPGRPs had the complete functions of effectors and partial functions of receptors from PGRPs. Further analyses showed that ApPGRPs from A. packardana might be involved in the endosymbiosis relationship between the host clam and endosymbiotic bacteria as a regulator. The results of these experiments suggested that ApPGRPs were involved in cold seep clams’ immune response. This study provides basic information for further research on the immune mechanisms of deep sea organisms.

Peptidoglycan recognition protein of Solen grandis (SgPGRP-S1) mediates immune recognition and bacteria clearance

Peptidoglycan recognition proteins (PGRPs) are indispensable molecules in innate immunity due to their prominent function in sensing and eliminating invading microorganisms. In the present study, a short type PGRP from razor clam Solen grandis (SgPGRP-S1) was recombinantly expressed and purified to investigate its potential function in innate immunity. As a pattern recognition receptor, recombinant SgPGRP-S1 (rSgPGRP-S1) specifically bind Lys-type and Dap-type peptidoglycan in vitro, but not lipopolysaccharide or β-glucan. The peptidoglycan binding ability of rSgPGRP-S1 resulted in significant agglutination activity against Gram-negative Escherichia coli and Listonella anguillarum, as well as Gram-positive Micrococcus luteus. Furthermore, rSgPGRP-S1 was bactericidal, significantly suppressing the growth of both E. coli and Gram-positive Staphylococcus aureus. The protein also exhibited strong amidase activity and degraded bacterial peptidoglycan in the presence of Zn²⁺, suggesting amidase activity might contribute to SgPGRP-S1 antibacterial activity. These results indicate SgPGRP-S1 is multifunctional in innate immunity, mediating both immune recognition and bacteria elimination.

Functional characterization of a short peptidoglycan recognition protein from Chinese giant salamander (Andrias davidianus)

Peptidoglycan (PGN) recognition proteins (PGRPs) are important pattern recognition receptors (PRRs) involved in immune defense against bacterial infections. In this study, a short PGRP (termed AdPGRP-S1) was cloned and functionally characterized from Chinese giant salamander (Andrias davidianus), the largest extant urodela amphibian species. AdPGRP-S1 was 184 aa in length and shared 38.7%-54.9% sequence identities with other vertebrates’ short PGRPs. It contained one typical PGRP domain at the C-terminal region and several conserved amino acid (aa) residues involved in amidase and PGN binding. AdPGRP-S1 was constitutively expressed in all tissues examined, with the highest expression level seen in spleen and intestine. It has been shown that AdPGRP-S1 could bind and degrade Lys-PGN and Dap-PGN. Further, AdPGRP-S1 had antibacterial activity against the Gram-negative bacteria, Edwardsiella tarda, and was able to trigger the activation of NF-κB signaling. These results demonstrated that AdPGRP-S1 possesses multiple functions in pathogen recognition, mediating ceullular signaling, and initiating antibacterial response. This is the first functional study of a salamander PGRP, providing insight to further understand the functional evolution of verterbates’ PGRPs.

Molecular and functional characterization of peptidoglycan-recognition protein SC2 (PGRP-SC2) from Nile tilapia (Oreochromis niloticus) involved in the immune response to Streptococcus agalactiae

PGRP-SC2, the member of PGRP family, plays an important role in regulation of innate immune response. In this paper, a PGRP-SC2 gene of Nile tilapia, Oreochromis niloticus (designated as On-PGRP-SC2) was cloned and its expression pattern under the infection of Streptococcus agalactiae was investigated. Sequence analysis showed main structural features required for amidase activity were detected in the deduced amino acid sequence of On-PGRP-SC2. In healthy tilapia, the On-PGRP-SC2 transcripts could be detected in all the examined tissues, with the most abundant expression in the muscle. When infected with S. agalactiae, there was a clear time-dependent expression pattern of On-PGRP-SC2 in the spleen, head kidney and brain. The assays for the amidase activity suggested that recombinant On-PGRP-SC2 protein had a Zn(2+)-dependent PGN-degrading activity. Moreover, our works showed that recombinant On-PGRP-SC2 protein could significantly reduce bacterial load in target organs attacked by S. agalactiae. These findings indicated that On-PGRP-SC2 may play important roles in the immune response to S. agalactiae in Nile tilapia.

A short-type peptidoglycan recognition protein from tongue sole (Cynoglossus semilaevis) promotes phagocytosis and defense against bacterial infection

Peptidoglycan recognition proteins (PGRPs) are members of the innate immune system that interact with bacteria by binding to bacterial peptidoglycan. In this study, we examined the expression and function of a short type of PGRP, CsPGRP-SC2, from tongue sole (Cynoglossus semilaevis). CsPGRP-SC2 contains 164 amino acid residues and shares 54.5%-65.3% overall sequence identities with other teleost PGRPs. CsPGRP-SC2 possesses an amidase domain with a conserved zinc binding site. CsPGRP-SC2 expression occurred in multiple tissues and was upregulated by bacterial and viral infection. Purified recombinant CsPGRP-SC2 (rCsPGRP-SC2) was able to bind and agglutinate Gram-positive bacteria in a Zn(2+)-dependent manner. rCsPGRP-SC2 enhanced the uptake of the bound bacteria by host phagocytes and reduced bacterial dissemination and colonization in host tissues. These results indicate that CsPGRP-SC2 is an innate immune factor that participates in host defense against bacterial infection.

Molecular and functional characterization of CD59 from Nile tilapia (Oreochromis niloticus) involved in the immune response to Streptococcus agalactiae

CD59, the major inhibitor of membrane attack complex, plays a crucial role in regulation of complement activation. In this paper, a CD59 gene of Nile tilapia, Oreochromis niloticus (designated as On-CD59) was cloned and its expression pattern under the stimulation of Streptococcus agalactiae was investigated. Sequence analysis showed main structural features required for complement-inhibitory activity were detected in the deduced amino acid sequence of On-CD59. In healthy Nile tilapia, the On-CD59 transcripts could be detected in all the examined tissues, with the most abundant expression in the brain. When immunized with inactivated S. agalactiae, there was a clear time-dependent expression pattern of On-CD59 in the skin, brain, head kidney, thymus and spleen, with quite different kinetic expressions. The assays for the complement-inhibitory activity suggested that recombinant On-CD59 protein had a species-selective inhibition of complement. Moreover, our works showed that recombinant On-CD59 protein may possess both binding activities to PGN and LTA and inhibiting activity of S. agalactiae. These findings indicated that On-CD59 may play important roles in the immune response to S. agalactiae in Nile tilapia.

Expression and functional characterization of PGRP6 splice variants in grass carp Ctenopharyngodon idella

Peptidoglycan recognition proteins (PGRPs), which are evolutionarily conserved pattern recognition receptors from insects to mammals, recognize bacterial PGN and function in antibacterial innate immunity. The existence of alternative splicing is a common feature for PGRP family. Here the splicing pattern from the splicing at the 5' end of PGRP6 gene was identified in a teleost fish, the grass carp (Ctenopharyngodon idella). Four splice variants of grass carp PGRP6 were designated as gcPGRP6a, gcPGRP6b, gcPGRP6c and gcPGRP6d, respectively. Real-time PCR revealed the different expression of these variants in fish individuals and CIK cell line in response to stimulation with different microbial ligands. Immunofluorescence microscopy and Western blotting showed that the splice variants are intracellular protein. Cell lysates from Epithelioma papulosum cyprini (EPC) cells transfected with gcPGRP6 splice variants are able to bind microbial PAMPs including Lys-type PGN from Staphylococcus aureus, DAP-type PGN from Bacillus subtilis, glucan, mannan, and microorganisms including Streptococcus dysgalactiae, Flavobacterium columnare and Saccharomyces cerevisiae. Moreover, overexpression of gcPGRP6 variants inhibited earlier stage growth of intracellular bacteria. The data also identified a specific role for gcPGRP6c variant in the positive regulation of cytolytic molecule perforin, and for gcPGRP6a, gcPGRP6b and gcPGRP6c variants in positive regulation of antimicrobial peptides (AMPs). However, the gcPGRP6d variant, which encoded basically only the PGRP domain, failed to induce the expression of perforin and AMPs. It is suggested that fish PGRP6 splice variants have common and variant-specific function in innate immune response.

Pathogen recognition receptors in channel catfish: IV. Identification, phylogeny and expression analysis of peptidoglycan recognition proteins

Peptidoglycan recognition proteins (PGRPs) can recognize bacterial cell wall (peptidoglycan) and activate innate immune system. In addition to its function as pathogen recognition receptors (PRRs), PGRPs are also involved in directly killing bacteria, and regulating multiple signaling pathways. Recently, we have reported catfish PRRs including nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs), retinoic acid inducible gene I (RIG-I) like receptors (RLRs), and Toll-like receptors (TLRs). In this study, we identified and characterized the PGRP gene family in channel catfish which included two members, PGLYRP-5 and PGLYRP-6. Phylogenetic analysis, syntenic analysis and protein structural analysis were conducted to determine their identities and evolutionary relationships. In order to gain insight into the roles of PGRPs in catfish innate immune responses, quantitative real-time PCR was used to investigate the expression profiles in catfish healthy tissues and after bacterial infection. Both PGLYRP-5 and PGLYRP-6 were ubiquitously expressed in all 12 healthy tissues, and most highly expressed in gill and spleen, respectively. Distinct expression patterns were observed for PGRPs after infection with Edwardsiella ictaluri and Flavobacterium columnare, both Gram-negative bacteria. After infection with E. ictaluri, both PGLYRP-5 and PGLYRP-6 were significantly down-regulated at a certain time-point, while both genes were generally up-regulated in the gill after infection with F. columnare. Collectively, these findings suggested that PGRPs may play complex roles in the host immune response to bacterial pathogens in catfish.

Molecular cloning and functional characterization of peptidoglycan recognition protein 6 in grass carp Ctenopharyngodon idella

Peptidoglycan recognition proteins (PGRPs) are pattern recognition molecules of innate immunity. In this study, a long-form PGRP, designated as gcPGRP6, was identified from grass carp Ctenopharyngodon idella. The deduced amino acid sequence of gcPGRP6 is composed of 464 residues with a conserved PGRP domain at the C-terminus. The gcPGRP6 gene consists of four exons and three introns, spacing approximately 2.7 kb of genomic sequence. Phylogenetic analysis demonstrated that gcPGRP6 is clustered closely with zebrafish PGLYRP6, and formed a long-type PGRP subfamily together with PGLYRP2 members identified in teleosts and mammals. Real-time PCR and Western blotting analyses revealed that gcPGRP6 is constitutively expressed in organs/tissues examined, and its expression was significantly induced in liver and intestine of grass carp in response to PGN stimulation and in CIK cells treated with lipoteichoic acid (LTA), polyinosinic polycytidylic acid (Poly I:C) and peptidoglycan (PGN). Immunofluorescence microscopy and Western blotting analyses revealed that gcPGRP6 is effectively secreted to the exterior of CIK cells. The over-expression of gcPGRP6 in CIK cells leads to the activation of NF-κB and the inhibition of intracellular bacterial growth. Moreover, cell lysates from CIK cells transfected with pTurbo-gcPGRP6-GFP plasmid display the binding activity towards Lys-type PGN from Staphylococcus aureus and DAP-type PGN from Bacillus subtilis. Furthermore, proinflammatory cytokine IL-2 and intracellular PGN receptor NOD2 had a significantly increased expression in CIK cells overexpressed with gcPGRP6. It is demonstrated that the PGRP6 in grass carp has a role in binding PGN, in inhibiting the growth of intracellular bacteria, and in activating NF-κB, as well as in regulating innate immune genes.

Rainbow trout peptidoglycan recognition protein has an anti-inflammatory function in liver cells

Peptidoglycan recognition proteins (PGRPs) are innate immune molecules that are structurally conserved through evolution in both invertebrate and vertebrate animals. PGRPs exert diverse host-defense functions both through direct antibacterial activity and through indirect effects, including the induction of antimicrobial peptides and the modulation of inflammation and immune responses. In this study, we identified the gene encoding a long form of PGRP (OmPGRP-L1) from the rainbow trout, Oncorhynchus mykiss, and investigated whether it has immunomodulating activity in a rainbow trout hepatoma cell line RTH-149 challenged with fish pathogenic bacteria. OmPGRP-L1 contains the conserved PGRP domain and the four Zn(2+)-binding amino acid residues required for amidase activity. In RTH-149 cells, OmPGRP-L1 expression was increased by bacterial stimulation. Loss-of-function and gain-of-function experiments indicated that OmPGRP-L1 is involved in the expression of pro-inflammatory cytokines. Silencing of OmPGRP-L1 in RTH-149 cells challenged with Edwardsiella tarda dramatically increased the expression of IL-1β and TNF-α. In contrast, overexpression of OmPGRP-L1 or its amidase-inactive mutant OmPGRP-L1(C472S) resulted in down-regulation of IL-1β and TNF-α expression. When overexpressed in RTH-149 cells, OmPGRP-L1 inhibited NF-κB activity with or without bacterial stimulation. Collectively, these findings suggest that OmPGRP-L1 has an anti-inflammatory function, independent of its amidase activity, possibly via NF-κB inhibition in liver cells.