A peptidoglycan recognition protein involved in immune recognition and immune defenses in Ruditapes philippinarum

Characterization of a novel CUB domain containing protein with immune functions in Mytilus galloprovincialis

CUB (Complement/Uegf/Bmp) domain is a structural motif present in many proteins with important functions in tissue development and immunity. CUB-domain-containing proteins (CDCPs) have been identified as immune-related molecules from bivalves. However, the knowledge about the role of CDCP in bivalves' immunity is still limited. In this study, a novel CDCP containing a signal peptide and only one CUB domain was characterized from Mytilus galloprovincialis and named Mg-CUB here. The tissue distribution and expression profiles of Mg-CUB gene after microbes' stimulation were determined. In addition, Mg-CUB was recombinantly expressed and the immune-related functions of rMg-CUB protein were tested. Mg-CUB exhibits a wide distribution in all tested tissues with the highest expression level was observed in adductor muscle followed by hemocytes. Infection with bacteria significantly up-regulated the expression level of Mg-CUB in the hemocytes, suggesting that Mg-CUB might be involved in host immune defense response. Recombinant Mg-CUB (rMg-CUB) exhibited relatively stronger antimicrobial activity against Gram-negative bacteria, as well as binding capacity and agglutination ability for the tested bacteria. In addition, rMg-CUB also showed binding ability with lipopolysaccharide and peptidoglycan. Injection of rMg-CUB protein significantly increased the expression level of TLR4, MyD88, and NF-κB in the hemocytes, indicating a potential role of Mg-CUB in the activation of the related signaling pathway. This study provided valuable data for understanding the biological roles of CUB domain as both immune effector and pattern recognition receptor (PRR) in the innate immune system of mollusks.

A Review on the Extraction, Structural Characterization, Function, and Applications of Peptidoglycan

Peptidoglycan (PGN) is the primary component of bacterial cell walls, consisting of linear glycan chains formed by alternating linkages of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) through glycosidic bonds. It exhibits biological activity in various aspects, making it a biologically significant macromolecule with extensive industrial application. This review aims to explore the latest research advancements in the extraction techniques, structural characterization, functions, and applications of PGN. The review compares the advantages and limitations of traditional chemical lysis methods with modern mechanical-assisted and bio-assisted extraction techniques, discusses chemical composition analysis techniques and structural characterization methods of PGN. The review emphasizes the potential of PGN in immune modulation, specific recognition, and adsorption functions. Furthermore, the review examines potential applications of PGN in vaccine development, the livestock industry, the removal of harmful substances, and protein bioprocessing. In the end, based on the current development trend, future research directions for PGN are proposed, including in-depth studies on the mechanisms of PGN in different hosts and its immunomodulatory effects in various disease models. It is expected that a comprehensive reference framework for the research and application of PGN will be provided through this review, offering ideas and directions for further development and utilization.

The Role of AaPGRP-LB in the Immune Response of Aedes albopictus Against Bacteria Infection

The initial phase of an insect's innate immune response to foreign pathogens is triggered by the identification of exogenous invaders, a mechanism facilitated by pattern recognition receptors. Among these receptors, peptidoglycan recognition proteins (PGRPs), abundant in insects, are essential components of the innate immune system. The roles of PGRPs have been extensively elucidated in Drosophila melanogaster; however, the mechanism underlying the immune response of Aedes albopictus to pathogens is unclear. Herein, we successfully cloned the full-length cDNA of a PGRP gene from Ae. albopictus, designated as the AaPGRP-LB gene. The open reading frame of AaPGRP-LB encodes 203 amino acids, including a secretion signal peptide and a canonical PGRP conserved domain. Multisequence alignment revealed that AaPGRP-LB possesses the amino acid residues essential for zinc binding and amidase activity. Molecular docking studies demonstrated that AaPGRP-LB exhibits a strong binding affinity for DAP-type and LYS-type peptidoglycan. The mRNA expression level of the AaPGRP-LB gene significantly increased after oral infection with Escherichia coli or Staphylococcus aureus. The purified recombinant AaPGRP-LB (rAaPGRP-LB) exhibited strong agglutination properties and demonstrated significant antimicrobial efficacy against E. coli and S. aureus in the presence of zinc ions. This study highlights the critical role of AaPGRP-LB in the immune response of Ae. albopictus. These findings provide a foundation for future research on mosquito immune pathways for innovative vector control and disease prevention strategies.

Molecular and functional characterization of short peptidoglycan recognition proteins in Vesicomyidae clam

Within cold seep environments, the Vesicomyidae clam emerges as a prevalent species, distinguished by its symbiotic relationship with microorganisms housed within its organ gill. Given the extreme conditions and the symbiotic nature of this association, investigating the host's immune genes, particularly immune recognition receptors, is essential for understanding their role in facilitating host-symbiotic interactions. Three short peptidoglycan recognition proteins (PGRPs) were identified in the clam. AmPGRP-S1, -S2, and -S3 were found to possess conserved amidase binding sites and Zn2+ binding sites. Quantitative Real-time PCR (qRT-PCR) analysis revealed differential expression patterns among the PGRPs. AmPGRP-S1 and AmPGRP-S2 exhibited elevated expression levels in the gill, while AmPGRP-S3 displayed the highest expression in the adductor muscle. Functional experiments demonstrated that recombinant AmPGRP-S1, -S2, and -S3 (rAmPGRPs) exhibited binding capabilities to both L-PGN and D-PGN (peptidoglycan). Notably, rAmPGRP-S1 and -S2 possessed Zn2+-independent amidase activity, while rAmPGRP-S3 lacked this enzymatic function. rAmPGRPs were shown to bind to five different bacterial species. Among these, rAmPGRP-S1 inhibited Escherichia coli and Bacillus subtilis, while rAmPGRP-S2 and -S3 inhibited Bacillus subtilis in the absence of Zn2+. In the presence of Zn2+, rAmPGRP-S1 and -S2 exhibited enhanced inhibitory activity against Staphylococcus aureus or Bacillus subtilis. These findings suggest that AmPGRPs may play a pivotal role in mediating the interaction between the host and endosymbiotic bacteria, functioning as PGN and microbe receptors, antibacterial effectors, and regulators of host-microbe symbiosis. These results contribute to our understanding of the adaptive mechanisms of deep-sea organisms to the challenging cold seep environments.

Antiviral function of peptidoglycan recognition protein in Spodoptera exigua (Lepidoptera: Noctuidae)

Peptidoglycan recognition proteins (PGRPs) are a class of molecules that play a critical role in insect immunity. Understanding the function of PGRPs is important to improve the efficiency of microbial insecticides. In this study, we investigated the role of PGRP-LB (a long type PGRP) in insect immunity against viruses using Spodoptera exigua and Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) as an insect-virus model. We cloned and identified a PGRP-LB gene from S. exigua; the gene consisted of 7 exons that encoded a polypeptide of 234 amino acids with a signal peptide and a typical amidase domain. Expression analysis revealed that the abundance of SePGRP-LB transcripts in the fat body was greater than in other tissues. Overexpression of SePGRP-LB resulted in a significant decrease of 49% in the rate of SeMNPV-infected cells. In addition, the multiplication of SeMNPV was significantly decreased: a decrease of 79% in the production of occlusion-derived virion (ODV), and a maximum decrease of 50% in the production of budded virion (BV). In contrast, silencing of SePGRP-LB expression by RNA interference resulted in a significant 1.65-fold increase in the rate of SeMNPV-infected cells, a significant 0.54-fold increase in ODV production, a maximum 1.57-fold increase in BV production, and the larval survival dropped to 21%. Our findings show that SePGRP-LB has an antiviral function against SeMNPV, and therefore this gene may provide a target for lepidopteran pest control using virus insecticides.

Molecular characterization of peptidoglycan recognition proteins from Mytilus coruscus

Mytilus shows great immune resistance to various bacteria from the living waters, indicating a complex immune recognition mechanism against various microbes. Peptidoglycan recognition proteins (PGRPs) play an important role in the defense against invading microbes via the recognition of the immunogenic substance peptidoglycan (PGN). Therefore, eight PGRPs were identified from the gill transcriptome of Mytilus coruscus. The sequence features, expression pattern in various organs and larval development stages, and microbes induced expression profiles of these Mytilus PGRPs were determined. Our data revealed the constitutive expression of PGRPs in various organs with relative higher expression level in immune-related organs. The expression of PGRPs is developmentally regulated, and most PGRPs are undetectable in larvae stages. The expression level of most PGRPs was significantly increased with in vivo microbial challenges, showing strong response to Gram-positive strain in gill and digestive gland, strong response to Gram-negative strain in hemocytes, and relative weaker response to fungus in the three tested organs. In addition, the function analysis of the representative recombinant expressed PGRP (rMcPGRP-2) confirmed the antimicrobial and agglutination activities, showing the immune-related importance of PGRP in Mytilus. Our work suggests that Mytilus PGRPs can act as pattern recognition receptors to recognize the invading microorganisms and the antimicrobial effectors during the innate immune response of Mytilus.

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.

Expression and functional characterization of peptidoglycan recognition protein-S6 involved in antibacterial responses in the razor clam Sinonovacula constricta

It has been recognized that peptidoglycan recognition proteins (PGRPs), structurally conserved molecules, play crucial roles in the innate immunity of invertebrate. However, few studies have been taken to explore their potential functions. In this study, a novel PGRP from the razor clam Sinonovacula constrict designated as ScPGRP-S6 was identified and characterized. The open reading frame (ORF) of ScPGRP-S6 was 666 bp in length, encoding a protein of 221 amino acid with a signal peptide (1-30) and a typical PGRP domain (39-187). The sequence alignment combined with phylogenetic analysis collectively confirmed that ScPGRP-S6 was a novel member belonging to PGRP-S family. The mRNA transcript of ScPGRP-S6 in the hepatopancreases was significantly up-regulated after peptidoglycan (PGN) stimulation, while it was moderately up-regulated after lipopolysaccharide (LPS) stimulation. The result of immunofluorescence detection demonstrated that the positive signal enhanced obviously after Vibrio parahaemolyticus challenge. Notably, the recombinant protein of ScPGRP-S6 (designed as rScPGRP-S6) exhibited high agglutination activity towards V. parahaemolyticus but weak to Staphylococcus aureus. Furthermore, rScPGRP-S6 showed strong amidase and antibacterial activity in the presence of Zn²⁺. Collectively, our results manifested that ScPGRP-S6 could act as a scavenger in the innate immune response of S. constricta.

Genome-wide identification and characteristic analysis of PGRP gene family in Tegillarca granosa reveals distinct immune response of the invasive pathogen

The peptidoglycan recognition proteins (PGRPs) are conserved innate immune molecular in invertebrates and vertebrates, which play important roles in immune system by recognize the peptidoglycans of bacterial cell walls. Although PGRPs have been extensively characterized in insects, a systematic analysis of PGRPs in bivalves is lacking. In the present study, the phylogenic relationships, gene structures and expression profiles of PGRPs in marine bivalves were analyzed. The results indicated that the most PGRPs of bivalves were predicted to degrade the peptidoglycans and prevent excessive immunostimulation of bacteria. In addition, the results of the present study showed that the protein diversity of PGRPs in most marine bivalves was mainly generated by the alternative splicing of genes, however the alternative splicing of PGRP gene family was absent in Tegillarca granosa. The differences of PGRPs might be related to the genetic and environmental differences of marine bivalves. Spatiotemporal expression profiling in T. granosa suggested that PGRPs play important roles in the immune response of invasive pathogens. The present study describes a comprehensive view of PGRPs in the blood clam T. granosa and provides a foundation for functional characterization of this gene family in innate immune of marine bivalves.

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.

Identification of peptidoglycan recognition proteins in hemocytes and kidney of common periwinkle Littorinalittorea

Peptidoglycan Recognition Proteins (PGRPs) are a diverse group of proteins involved in innate immunity. In particular, PGRPs have been shown to participate in immune pattern recognition in various mollusks. However, they have not been described in Caenogastropoda, a large molluscan group comprising sea, freshwater and land snails. In this study, four short PGRPs with molecular weights ranging from 21 to 34 kDa and their isoforms were identified and structurally characterized in the kidney and hemocytic transcriptomes of a caenogastropod mollusk Littorina littorea. All of them (LlPGRP1-4) are secretory, possess a signal peptide and a characteristic N-terminal N-acetylmuramoyl-l-alanine amidase (Ami) domain with conserved Zn²⁺ binding- and amidase catalytic sites. The shortest proteins, LlPGRP1 and LlPGRP2, have no additional conserved motifs on the N-terminus. In longer and most abundantly expressed LlPGRP3 and LlPGRP4 the Ami-domain is combined with an N-terminal SH3-domain and a cysteine-rich motif, respectively. Expression analysis showed that LlPGRPs of the common periwinkle were uninvolved in the immune response to infection with trematode Himasthla elongata though they might act in antibacterial defense.

A role of peptidoglycan recognition protein in mediating insecticide detoxification in Glyphodes pyloalis

Glyphodes pyloalis Walker has become one of the most significant mulberry pests, and it has caused serious economic losses in major mulberry growing regions in China. Peptidoglycan recognition proteins (PGRPs) are responsible for initiating and regulating immune signalling pathways in insects. However, their roles responding to chemical pesticides is still less known. This study aimed to investigate the possible detoxication function of GpPGRP-S2 and GpPGRP-S3 in G. pyloalis in response to chlorfenapyr and phoxim. The chlorfenapyr and phoxim treatment significantly induced the expression level of GpPGRP-S3 at 48 h. In addition, the expression levels of GpPGRP-S2 and GpPGRP-S3 in the chlorfenapyr/phoxim treatment group were significantly higher in midgut than those in the control group at 48 h. The results of the survival experiment showed that silencing either GpPGRP-S2 or GpPGRP-S3 would not influence the survival rate of G. pyloalis which treated with phoxim, however, silencing GpPGRP-S2 or GpPGRP-S3 would cause G. pyloalis to be more easily killed by chlorfenapyr. The expression of carboxylesterase GpCXE1 was significantly induced by chlorfenapyr/phoxim treatment, while it was suppressed once silenced GpPGRP-S2 followed with chlorfenapyr treatment or silenced GpPGRP-S3 followed with phoxim treatment. These results might suggest that under the chlorfenapyr/phoxim treatment condition, the connection between GpPGRPs and detoxification genes in insect was induced to maintain physiological homeostasis; and these results may further enrich the mechanisms of insects challenged by insecticides.

Molecular and functional identification of a short-type peptidoglycan recognition protein, PGRP-S, in the Chinese soft-shelled turtle Pelodiscus sinensis

Peptidoglycan recognition proteins (PGRPs), which are discovered in invertebrates and vertebrates, play an important role in antibacterial immunity. However, the function of PGRPs is largely uninvestigated in reptiles. In the present study, a short-type PGRP gene, designed as C-turtle-PGRP-S, was identified in the Chinese soft-shelled turtle, Pelodiscus sinensis. The C-turtle-PGRP-S contains a highly conserved PGRP domain and has close relationship with PGRP-S orthologues in other species according to sequence and phylogenetic analyses. C-turtle-PGRP-S gene was constitutively expressed in all detected tissues and was induced by Edwardsiella tarda. Additionally, recombinant C-turtle-PGRP-S showed PGN binding activity and antibacterial function against E. tarda. Therefore, it is suggested that the function of PGRP-S is likely to be conserved in reptile vertebrates, as observed in other vertebrates, shedding light on the evolutionary conservation of PGRPs.

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.