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

Molecular characterization and gene expression of pattern recognition receptors in brown-marbled grouper (Epinephelus fuscoguttatus) fingerlings responding to vibriosis infection

The pathogen recognition system involves receptors and genes that play a crucial role in activating innate immune response in brown-marbled grouper (Epinephelus fuscoguttatus) as a control agent against various infections including vibriosis. Here, we report the molecular cloning of partial open reading frames, sequences characterization, and expression profiles of Pattern Recognition Receptors (PRRs) in brown-marbled grouper. The PRRs, namely pglyrp5, tlr5, ctlD, and ctlE in brown-marbled grouper, possess conserved domains and showed shared evolutionary relationships with other fishes, humans, mammals, birds, reptilians, amphibians, and insects. In infection experiments, up to 50% mortality was found in brown-marbled grouper fingerlings infected with Vibrio alginolyticus compared to 27% mortality infected Vibrio parahaemolyticus and 100% survival of control groups. It is also demonstrated that all four PRRs had higher expression in samples infected with V. alginolyticus compared to V. parahaemolyticus. This PRRs gene expression analysis revealed that all four PRRs expressed rapidly at 4-h post-inoculation even though the Vibrio count was only detected earliest at 12-h post-inoculation in samples. The highest expression recorded was from V. alginolyticus inoculated fish spleen with up to 73-fold change for pglyrp5 gene, followed by 14 to 38-fold expression for the same treatment in spleen, head kidney, and blood samples for other PRRs, namely tlr5, ctlD, and ctlE genes. Meanwhile less than a 10% increase in expression of all four genes was detected in spleen, head kidney, and blood samples inoculated with V. parahaemolyticus. These findings indicated that pglyrp5, tlr5, ctlD, and ctlE play important roles in the early immune response to vibriosis infected, brown-marbled grouper fingerlings.

The role of a novel secretory peptidoglycan recognition protein with antibacterial ability from the Chinese Oak Silkworm Antheraea pernyi in humoral immunity

Peptidoglycan recognition proteins (PGRPs) are a family of pattern recognition receptors that play a critical role in the immune response of invertebrates and vertebrates. Herein, the short ApPGRP-D gene was cloned from the model lepidopteran Antheraea pernyi. Quantitative PCR (qPCR) confirmed that ApPGRP-D is an immune-related protein and that the expression of ApPGRP-D can be induced by microorganisms. ApPGRP-D is a broad-spectrum pattern recognition protein that activates the prophenoloxidase cascade activation system and promotes the agglutination of microbial cells. Likely due to its amidase activity, ApPGRP-D can inhibit the growth of E. coli and S. aureus. In addition, we demonstrated for the first time that zinc ions, as important metal coenzymes, could promote multiple functions of ApPGRP-D but not its amidase activity.

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.

Insight into the molecular structure and function of peptidoglycan recognition protein SC2 (PGRP-SC2) from Amphiprion clarkii: Investigating the role in innate immunity

Peptidoglycan recognition proteins (PGRPs) belong to the pattern recognition receptor (PRR) family and are conserved from insects to mammals. PGRPs show specific binding abilities to peptidoglycans (PGNs) in various microbes. In this study, molecular and functional analyses of PGRP-SC2 from Amphiprion clarkii (AcPGRP-SC2) were conducted. The 492 bp ORF of AcPGRP-SC2 encoded a protein of 164 amino acids with a molecular weight of 17.58 kDa and pI of 8.9. The PGRP superfamily domain was identified from the protein sequence of AcPGRP-SC2 and sequence similarities were observed with homologous proteins. Quantitative polymerase chain reaction (qPCR) analysis revealed that AcPGRP-SC2 transcripts were ubiquitously expressed in all tested tissues, with high levels in the skin, and transcript expression was significantly modulated by immune stimulation with lipopolysaccharide (LPS), Polyinosinic:polycytidylic acid (poly I:C), and Vibrio harveyi post-immune challenge. Recombinant AcPGRP-SC2 with the maltose-binding protein fusion (rAcPGRP-SC2) was used to evaluate LPS-, PGN-, and bacterial-binding activities and to conduct bacterial agglutination assays, and the results demonstrated that AcPGRP-SC2 exhibited bacterial recognition, binding, and colonization abilities to a range of Gram-positive and Gram-negative bacterial strains. Moreover, rAcPGRP-SC2-pre-treated Fat Head Minnow (FHM) cells exhibited significant upregulation in NF-ĸB1, NF-ĸB2, and stat3 expression upon treatment with killed bacteria. Taken together, our findings suggest that AcPGRP-SC2 plays an important role in the immune response against microbial pathogens in A. clarkii.

Molecular and Functional Characterization of a Short-Type Peptidoglycan Recognition Protein, Ct-PGRP-S1 in the Giant Triton Snail Charonia tritonis

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 potential function of PGRPs in the giant triton snail Charonia tritonis. In this study, a short-type PGRP gene (termed Ct-PGRP-S1) was identified in C. tritonis. Ct-PGRP-S1 was predicted to contain several structural features known in PGRPs, including a typical PGRP domain (Amidase_2) and Src homology-3 (SH3) domain. The Ct-PGRP-S1 gene was constitutively expressed in all tissues examined except in proboscis, with the highest expression level observed in the liver. As a typical PRR, Ct-PGRP-S1 has an ability to degrade peptidoglycan (PGN) and was proven to have non-Zn2+-dependent amidase activity and antibacterial activity against Vibrioalginolyticus and Staphylococcus aureus. It is the first report to reveal the peptidoglycan recognition protein in C. tritonis, and these results suggest that peptidoglycan recognition protein Ct-PGRP-S1 is an important effector of C. tritonis that modulates bacterial infection resistance of V. alginolyticus and S. aureus, and this study may provide crucial basic data for the understanding of an innate immunity system of C. tritonis.

A short peptidoglycan recognition protein protects Boleophthalmus pectinirostris against bacterial infection via inhibiting bacterial activity

Peptidoglycan recognition proteins (PGRPs) belong to a member of pattern-recognition receptors (PRRs), which proposed as antibacterial protein. The present study investigated the antibacterial effect of BpPGRP5 in great blue-spotted mudskipper (Boleophthalmus pectinirostris). BpPGRP5 transcript was detected in all tested tissues with the highest expression level in spleen, and its expression was significantly upregulated in spleen, intestine, and kidney following Aeromonas veronii infection. rBpPGRP5 was found to interact with several polysaccharides and bacteria, including Gram-negative bacteria (Escherichia coli and A. veronii) and Gram-positive bacteria (Listeria monocytogenes and Staphylococcus aureus). rBpPGRP5 inhibited the proliferation of E. coli, S. aureus, L. monocytogenes, and A. veronii in a Zn²⁺-dependent manner. Furthermore, in vivo studies revealed that intraperitoneal injection of rBpPGRP5 improved the survival rate of A. veronii-infected B. pectinirostris, accompanied by decreased bacterial load in the blood, kidney, intestine, and spleen. Taken together, our results indicated that BpPGRP5 is an antimicrobial protein that protects B. pectinirostris against bacterial infection.

Molecular mechanism and potential application of bacterial infection in the silkworm, Bombyx mori

As a representative species of Lepidoptera, Bombyx mori has been widely studied and applied. However, bacterial infection has always been an important pathogen threatening the growth of silkworms. Bombyx mori can resist various pathogenic bacteria through their own physical barrier and innate immune system. However, compared with other insects, such as Drosophila melanogaster, research on the antibacterial mechanism of silkworms is still in its infancy. This review systematically summarized the routes of bacterial infection in silkworms, the antibacterial mechanism of silkworms after ingestion or wounding infection, and the intestinal bacteria and infection of silkworms. Finally, we will discuss silkworms as a model animal for studying bacterial infectious diseases and screening antibacterial drugs.

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.

Tenebrio molitor PGRP-LE Plays a Critical Role in Gut Antimicrobial Peptide Production in Response to Escherichia coli

Invading pathogens are recognized by peptidoglycan recognition proteins (PGRPs) that induce translocation of NF-κB transcription proteins and expression of robust antimicrobial peptides (AMPs). Tenebrio molitor PGRP-LE (TmPGRP-LE) has been previously identified as a key sensor of Listeria monocytogenes infection. Here, we present that TmPGRP-LE is highly expressed in the gut of T. molitor larvae and 5-day-old adults in the absence of microbial infection. In response to Escherichia coli and Candida albicans infections, TmPGRP-LE mRNA levels are significantly upregulated in both the fat body and gut. Silencing of TmPGRP-LE by RNAi rendered T. molitor significantly more susceptible to challenge by E. coli infection and, to a lesser extent, Staphylococcus aureus and C. albicans infections. Reduction of TmPGRP-LE levels in the larval gut resulted in downregulation of eight AMP genes following exposure to E. coli, S. aureus, and C. albicans. However, the transcriptional levels of AMPs more rapidly reached a higher level in the dsEGFP-treated larval gut after challenge with E. coli, which may suggest that AMPs induction were more sensitive to E. coli than S. aureus and C. albicans. In addition, TmPGRP-LE RNAi following E. coli and C. albicans challenges had notable effects on TmRelish, TmDorsal X1 isoform (TmDorX1), and TmDorX2 expression level in the fat body and gut. Taken together, TmPGRP-LE acts as an important gut microbial sensor that induces AMPs via Imd activation in response to E. coli, whereas involvement of TmPGRP-LE in AMPs synthesize is barely perceptible in the hemocytes and fat body.

Molecular cloning and functional characterization of a short peptidoglycan recognition protein from triangle-shell pearl mussel (Hyriopsis cumingii)

Peptidoglycan (PGN) is an important target of recognition in invertebrate innate immunity. PGN recognition proteins (PGRPs) are responsible for PGN recognition. In this study, we cloned and functionally analyzed a short PGRP (HcPGRP2) from the triangle-shell pearl mussel Hyriopsis cumingii. The full-length cDNA sequence of HcPGRP2 gene was 1185 bp containing an open reading frame of 882 bp encoding a 293 amino acid protein. HcPGRP2 was predicted to have two SH3b domains and a conserved C-terminal PGRP domain. Quantitative real-time RT-PCR showed that HcPGRP2 was expressed in all examined tissues and its expression was induced most significantly by Staphylococcus aureus and Vibrio parahaemolyticus in the hepatopancreas and gills. RNA interference by siRNA results revealed that HcPGRP2 was involved in the regulation of whey acidic protein, theromacin, and defensin expression. As a pattern-recognition receptor, recombinant HcPGRP2 (rHcPGRP2) protein can bind and agglutinate (Ca²⁺ dependent) all tested bacteria. rHcPGRP2 exhibited specific binding to PGN but not to lipopolysaccharide. Moreover, rHcPGRP2 inhibited the growth activities of S. aureus and V. parahaemolyticus in vitro and accelerated the clearance of V. parahaemolyticus in vivo. Overall, our results indicated that HcPGRP2 may play an important role in the antibacterial immune mechanisms of H. cumingii.

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.

Molecular characterization, expression and functional analysis of peptidoglycan recognition protein-SC2 from rock bream, Oplegnathus fasciatus

Peptidoglycan recognition proteins are members of the family of pattern recognition receptors (PRRs), that play important roles in the recognition of peptidoglycan and various biological processes. In this study, we have characterized peptidoglycan recognition protein-SC2 (PGRP-SC2) in rock bream (Oplegnathus fasciatus) (RbPGRP-SC2) and analysed its expression in various tissues after pathogen challenge. A sequence alignment revealed that the residues essential to zinc binding of the deduced protein were highly conserved among all the organisms. Phylogenetic analysis revealed that RbPGRP-SC2 is most closely related to the large yellow croaker PGRP-SC2. RbPGRP-SC2 was ubiquitously expressed in all tissues analysed, predominantly distributed in muscle and skin. After challenge with microbial pathogens (Edwardsiella piscicida), Streptococcus iniae or red seabream iridovirus [RSIV]), RbPGRP-SC2 was up-regulated in all the tissues examined, especially in liver. We produced recombinant RbPGRP-SC2 (rRbPGRP-SC2) using an Escherichia coli expression system. The rRbPGRP-SC2 had agglutination activity towards both Gram-negative (E. piscicida) and Gram-positive bacteria (S. iniae). In addition, rRbPGRP-SC2 induced leukocyte apoptosis and promoted leukocyte phagocytosis. These results suggest that the RbPGRP-SC2 plays an important role in the immune system and in maintaining cellular homeostasis of rock bream.

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.

Two short peptidoglycan recognition proteins from Crassostrea gigas with similar structure exhibited different PAMP binding activity

Peptidoglycan recognition protein (PGRP) is an essential molecule in innate immunity for both invertebrates and vertebrates, owing to its prominent ability in specifically recognizing bacterial peptidoglycan (PGN) and eliminating the invading bacteria. In the present study, the full length cDNA of two PGRP genes, CgPGRPS2 and CgPGRPS4, were cloned from oyster Crassostrea gigas. Their amino acid sequences both contained one signal peptide, one typical PGRP/amidase domain with conserved catalytic residues responsible for amidase activity (55H, 90Y, 164H, 172C in CgPGRPS2, and 98H, 133Y, 207H, 215C in CgPGRPS4), and specific PGN recognition (84R, 85W, 104R, 109V in CgPGRPS2, and 127G, 128W, 147R, 152V in CgPGRPS4), and they shared 55.9% sequence similarity. The mRNA transcripts of CgPGRPS2 and CgPGRPS4 were constitutively expressed in all the examined tissues, including haemocytes, hepatopancreas, mantle, gonad, heart, adductor muscle and gill, with the highest expression level in adductor muscle and hepatopancreas, respectively. Both CgPGRPS2 and CgPGRPS4 proteins were mainly localized in the cytoplasma. The recombinant protein of CgPGRPS2 (rCgPGRPS2) could bind lipopolysaccharide (LPS), PGN and mannan (Man), as well as various microorganisms including Gram-negative bacteria Escherichia coli, Vibrio anguillarum, Gram-positive bacteria Staphylococcus aureus and fungi Yarrowia lipolytica. The recombinant protein of CgPGRPS4 (rCgPGRPS4) exhibited higher binding affinity to PGN, lower binding affinity to LPS, while no binding activity to Man and Y. lipolytica. The results indicated that CgPGRPS2 and CgPGRPS4 could function as pattern recognition receptors (PRR) in the innate immune response of oyster, and they exhibited a certain degree of functional differentiation in recognition of Man.

Involvement of PGRP-SC2 from Artemia sinica in the innate immune response against bacteria and expression pattern at different developmental stages

Peptidoglycan-recognition protein-SC2 precursor-like protein (PGRP-SC2) is a vital protein in innate immunity with a vita role in response to bacteria challenge in invertebrates. Here, a 678-bp full-length cDNA of pgrp-sc2 from A. sinica was obtained containing a 558-bp open reading frame encoding 185 amino acids with a calculated molecular mass of 19.6 kDa. The predicted protein contains a PGRP and an Amidase2 domain, indicating that PGRP-SC2 is a PGRP family member and has N-acetylmuramoyl-l-alanine amidase activity. The expression and localization of pgrp-sc2/PGRP-SC2 in A.sinica during embryonic development and bacterial challenge were determined by qPCR, WB and ISH. During different A. sinica embryonic development stages, the expression level of pgrp-sc2/PGRP-SC2 was most highly expressed at 0 and 5 h and after challenge by Gram-positive bacteria, it increased with increasing bacterial concentrations, indicating that it plays a vital role in A. sinica early embryonic development and innate immunity.

CsPTX1, a pentraxin of Cynoglossus semilaevis, is an innate immunity factor with antibacterial effects

Pentraxin 1 (PTX1) is a member of the pentraxin protein family, which plays important roles in the innate immunity of vertebrates. In fish, the biological function of PTX1 is essentially unknown. In this study, we examined the expression and function of a PTX homologue (CsPTX1) from the tongue sole, Cynoglossus semilaevis. CsPTX1 contains 223 amino acids and shares 49.3%-38.8% overall sequence identity with other known fish pentraxins. CsPTX1 is expressed in multiple tissues and is upregulated by bacterial and viral infection. CsPTX1 contains a pentraxin domain, which is known to bind extracellular antigens, and recombinant CsPTX1 (rCsPTX1) bound a wide range of Gram-positive and Gram-negative bacteria. rCsPTX1 also agglutinated all the bacteria tested in a Ca(2+)-dependent manner and the agglutinating capacity of rCsPTX1 was abolished in the absence of calcium. As well as its ability to agglutinate bacterial cells, rCsPTX1 displayed apparent bacteriostatic activity against Pseudomonas fluorescens in vitro by influencing the permeability of the microbial envelope. When introduced in vivo, rCsPTX1 enhanced the host's resistance to bacterial infection. These results indicate that CsPTX1 is a classic pattern recognition molecule that defends C. semilaevis against bacterial infection.

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.