A galectin from Eriocheir sinensis functions as pattern recognition receptor enhancing microbe agglutination and haemocytes encapsulation

Molluscan pleiotropic FADD involved in innate immune signaling and induces apoptosis

Fas-associated protein with death domain (FADD) was initially identified as a crucial adaptor protein in the apoptotic pathway mediated by death receptor (DR). Subsequently, many studies have confirmed that FADD plays a vital role in innate immunity and inflammatory responses in animals. However, the function of this pleiotropic molecule in mollusk species has not been well explored. In this study, we successfully verified the gene sequence of FADD in the Zhikong scallop (Chlamys farreri) and designated it as CfFADD. The CfFADD protein contains a conserved death effector and death domains. Phylogenetic analysis showed that CfFADD is a novel addition to the molluscan FADD family with a close evolutionary relationship with molluscan FADD subfamily proteins. CfFADD mRNA expression in various scallop tissues was significantly induced by challenge with pathogen-associated molecular patterns (lipopolysaccharide, peptidoglycan, and poly(I:C)), suggesting its role in innate immunity in scallops. Co-immunoprecipitation showed that CfFADD interacted with the scallop DR (tumor necrosis factor receptor) and a signaling molecule involved in the Toll-like receptor pathway (interleukin-1 receptor-associated kinase), confirming that CfFADD may be involved in DR-mediated apoptosis and innate immune signaling pathways. Further studies showed that CfFADD interacted with CfCaspase-8 and activated caspase-3. HEK293T cells exhibited distinct apoptotic features after transfection with a CfFADD-expression plasmid, suggesting a functional DR-FADD-caspase apoptotic pathway in scallops. Overexpression of CfFADD led to a significant dose-dependent activation of interferon β and nuclear factor-κB reporter genes, demonstrating the key role of CfFADD in innate immunity. In summary, our research has confirmed the critical roles of CfFADD in innate immunity and apoptosis and provides valuable information for developing comparative immunology theories.

A novel lectin with a distinct Gal_Lectin and CUB domain mediates haemocyte phagocytosis in oyster Crassostrea gigas

Invertebrate lectins exhibit structural diversity and play crucial roles in the innate immune responses by recognizing and eliminating pathogens. In the present study, a novel lectin containing a Gal_Lectin, a CUB and a transmembrane domain was identified from the Pacific oyster Crassostrea gigas (defined as CgGal-CUB). CgGal-CUB mRNA was detectable in all the examined tissues with the highest expression in adductor muscle (11.00-fold of that in haemocytes, p < 0.05). The expression level of CgGal-CUB mRNA in haemocytes was significantly up-regulated at 3, 24, 48 and 72 h (8.37-fold, 12.13-fold, 4.28-fold and 10.14-fold of that in the control group, respectively) after Vibrio splendidus stimulation. The recombinant CgGal-CUB (rCgGal-CUB) displayed binding capability to Mannan (MAN), peptidoglycan (PGN), D-(+)-Galactose and L-Rhamnose monohydrate, as well as Gram-negative bacteria (Escherichia coli, V. splendidus and Vibrio anguillarum), Gram-positive bacteria (Micrococcus luteus, Staphylococcus aureus, and Bacillus sybtilis) and fungus (Pichia pastoris). rCgGal-CUB was also able to agglutinate V. splendidus, and inhibit V. splendidus growth. Furthermore, rCgGal-CUB exhibited the activities of enhancing the haemocyte phagocytosis towards V. splendidus, and the phagocytosis rate of haemocytes was descended in blockage assay with CgGal-CUB antibody. These results suggested that CgGal-CUB served as a pattern recognition receptor to bind various PAMPs and bacteria, and enhanced the haemocyte phagocytosis towards V. splendidus.

A molluscan IRF interacts with IKKα/β family protein and modulates NF-κB and MAPK activity

Interferon regulatory factor (IRF) family proteins are key transcription factors involved in vital physiological processes such as immune defense. However, the function of IRF in invertebrates, especially in marine shellfish is not clear. In this study, a new IRF gene (CfIRF2) was identified in the Zhikong scallop, Chlamys farreri, and its immune function was analyzed. CfIRF2 has an open reading frame of 1107 bp encoding 368 amino acids. The N-terminus of CfIRF2 consists of a typical IRF domain, with conserved amino acid sequences. Phylogenetic analysis suggested close evolutionary relationship with shellfish IRF1 subfamily proteins. Expression pattern analysis showed that CfIRF2 mRNA was expressed in all tissues, with the highest expression in the hepatopancreas and gills. CfIRF2 gene expression was substantially enhanced by a pathogenic virus (such as acute viral necrosis virus) and poly(I:C) challenge. Co-immunoprecipitation assay identified CfIRF2 interaction with the IKKα/β family protein CfIKK1 of C. farreri, demonstrating a unique signal transduction mechanism in marine mollusks. Moreover, CfIRF2 interacted with itself to form homologous dimers. Overexpression of CfIRF2 in HEK293T cells activated reporter genes containing interferon stimulated response elements and NF-κB genes in a dose-dependent manner and promoted the phosphorylation of protein kinases (JNK, Erk1/2, and P38). Our results provide insights into the functions of IRF in mollusks innate immunity and also provide valuable information for enriching comparative immunological theory for the prevention of diseases in scallop farming.

Molecular characterization and functional analysis of galectin-1 from silver pomfret (Pampus argenteus)

Galectins, as members of lectin families, exhibit a high affinity for β-galactosides and play diverse roles in biological processes. They function as pattern recognition receptors (PRRs) with important roles in immune defense. In this study, galectin-1, designated as SpGal-1, was identified and characterized from silver pomfret (Pampus argenteus). The SpGal-1 comprises an open reading frame (ORF) spanning 396 base pairs (bp) and encodes a deduced amino acid (aa) sequence containing a single carbohydrate recognition domain (CRD). Sublocalization analysis revealed that SpGal-1 was mainly expressed in the cytoplasm. The mRNA transcripts of SpGal-1 were ubiquitously detected in various tissues, with a higher expression level in the intestine. In addition, when exposed to Photobacterium damselae subsp. damselae (PDD) infection, both the liver and head kidney exhibited significantly increased SpGal-1 mRNA expression. The recombinant protein of SpGal-1 (named as rSpGal-1) demonstrated hemagglutination against red blood cells (RBCs) from Larimichthys crocea and P. argenteus in a Ca2+ or β-Mercaptoethanol (β-ME)-independent manner. Notably, rSpGal-1 could bind with various pathogen-associated molecular patterns (PAMPs) including D-galactose, D-mannose, lipopolysaccharide (LPS), and peptidoglycan (PGN), with highest affinity to PGN. Moreover, rSpGal-1 effectively interacted with an array of bacterial types encompassing Gram-positive bacteria (Staphylococcus aureus and Nocardia seriolae) and Gram-negative bacteria (PDD and Escherichia coli, among others), with the most robust binding affinity towards PDD. Collectively, these findings highlight that SpGal-1 is a crucial PRR with involvement in the host immune defense of silver pomfret.

A galectin-9 involved in the microbial recognition and haemocyte autophagy in the Pacific oyster Crassostrea gigas

Galectin-9 is a tandem-repeat type member of galectin family participating in various immune responses, such as cell agglutination, phagocytosis, and autophagy. In the present study, a tandem repeat galectin-9 (defined as CgGal-9) was identified from Pacific oyster Crassostrea gigas, which consisted of two conserved carbohydrate recognition domains (CRDs) joined by a linker peptide. CgGal-9 was closely clustered with CaGal-9 from C. angulata, and they were assigned into the branch of invertebrate galectin-9s in the phylogenetic tree. The mRNA transcripts of CgGal-9 were detected in all the tested tissues, with the highest expression level in haemocytes. The mRNA expressions of CgGal-9 in haemocytes increased significantly after lipopolysaccharide (LPS) and Vibrio splendidus stimulation. The recombinant CgGal-9 was able to bind all the examined pathogen-associated molecular patterns (LPS, peptidoglycan, and mannose) and microbes (V. splendidus, Escherichia coli, Micrococcus luteus, Staphylococcus aureus, Bacillus subtilis, and Pichia pastoris), and agglutinated most of them in the presence of Ca2+. In CgGal-9-RNAi oysters, the mRNA expressions of autophagy related genes (CgBeclin1, CgATG5, CgP62 and CgLC3) in haemocytes decreased significantly while that of CgmTOR increased significantly at 3 h after V. splendidus stimulation. The autophagy level and mRNA expressions of autophagy related genes decreased in haemocytes after CgGal-9 was blocked by the corresponding antibody. These results revealed that CgGal-9 was able to bind different microbes and might be involved in haemocyte autophagy in the immune response of oyster.

Identification of two galectin-4 proteins (PcGal4-L and PcGal4-L-CRD) and their function in AMP expression in Procambarus clarkii

Galectins, a family of lectins that bind to β-galactoside, possess conserved carbohydrate recognition domains (CRDs) and play a crucial role in recognizing and eliminating pathogens in invertebrates. Two galectin-4 genes (PcGal4) isoforms, named PcGal4-L and PcGal4-L-CRD, were cloned from the cDNA library of Procambarus clarkia in our study. PcGal4-L contains an open reading frame (ORF, 1089 bp), which encodes a protein consisting of 362 amino acids including a single CRD and six low complexity regions. The full-length cDNA of PcGal4-L-CRD contains a 483 bp ORF that encodes a protein of 160 amino acids, with a single CRD and a low-complexity region. The difference between the two PcGal4 isoforms is that PcGal4-L has 202 additional amino acids after the CRD compared to the PcGal4-L-CRD. These two isoforms are grouped together with other galectins from crustaceans through phylogenetic analysis. Further study revealed that total PcGal4 (including PcGal4-L and PcGal4-L-CRD) was primarily expressed in the muscle, gills and intestine. The mRNA levels of total PcGal4 in gills and hemocytes were significantly induced after challenge with Aeromonas hydrophila. Both recombinant PcGal4-L and its spliced isoform, PcGal4-L-CRD, could directly bind to lipopolysaccharides, peptidoglycan and five tested microorganisms, inducing a wide spectrum of microbial agglutination. The spliced isoform PcGal4-L-CRD showed a stronger binding ability than PcGal4-L. In addition, when the PcGal4 was knockdown, transcriptions of seven antimicrobial peptides (AMPs) genes (ALF5, ALF6, ALF8, CRU1, CRU2, CRU3 and CRU4) in gills and seven AMPs genes (ALF5, ALF6, ALF8, ALF9, CRU1, CRU3 and CRU4) in hemocytes were significantly decreased. Meanwhile, the survival rate of P. clarkii decreased in the PcGal4-dsRNA group. In summary, these results indicate that PcGal4 can mediate the innate immunity in P. clarkii by bacterial recognition and agglutination, as well as regulating AMP expression, thus recognition and understanding of the functions of galectin in crustaceans in immune resistance.

Molecular characterization and expression analysis of a QM protein gene in Chinese mitten crab Eriocheir sinensis

QM protein was previously discovered as a tumor suppressor, and numerous studies have shown that QM protein also played important roles in the immune responses. To investigate the potential roles of the QM protein gene in Eriocheir sinensis, the QM protein gene (designated as EsQM) has been cloned from E. sinensis using the rapid amplification of cDNA ends (RACE) technique. The cDNA of EsQM is 781 bp in length, consisting of a 654 bp open reading frame (ORF), encoding 219 amino acids, a 27 bp 5' untranslated region (UTR) and a 94 bp 3' UTR. The EsQM protein has a calculated molecular weight of 25.4 kDa and a theoretical isoelectric point of 10.10. The deduced protein sequence of EsQM contains a Ribosomal_L16 domain, an SH3-binding motif, an N-acylation site, two putative antibiotic binding sites, two putative protein kinase C phosphorylation sites, and two amidation sites. EsQM is extremely conserved and exhibits more than 85% similarities to previously identified arthropod QM protein genes. By real-time quantitative PCR (qPCR) analysis, we found that EsQM mRNA transcripts were detectable in all the examined tissues, with the highest expression in hemocytes. The mRNA expression of EsQM in hemocytes was significantly upregulated after the stimulation of Aeromonas hydrophila or polybrominated diphenyl ether-47 (BDE-47). Moreover, EsQM mRNA expression in hemocytes responded more quickly and lasted longer when stimulated by A.hydrophila than BDE-47. Thus, EsQM can respond to bacterial infection and environmental pollution, and might be involved in the defense mechanism to both biological and non-biological stimulation of arthropods.

Molecular Characterization of Galectin-3 in Large Yellow Croaker Larimichthys crocea Functioning in Antibacterial Activity

Galectins are proteins that play a crucial role in the innate immune response against pathogenic microorganisms. Previous studies have suggested that Galectin-3 could be a candidate gene for antibacterial immunity in the large yellow croaker Larimichthys crocea. In this study, we cloned the Galectin-3 gene in the large yellow croaker, and named it LcGal-3. The deduced amino acid sequence of LcGal-3 contains a carbohydrate recognition domain with two conserved β-galactoside binding motifs. Quantitative reverse transcription PCR (qRT-PCR) analysis revealed that LcGal-3 was expressed in all the organs/tissues that were tested, with the highest expression level in the gill. In Larimichthys crocea kidney cell lines, LcGal-3 protein was distributed in both the cytoplasm and nucleus. Moreover, we found that the expression of LcGal-3 was significantly upregulated upon infection with Pseudomonas plecoglossicida, as demonstrated by qRT-PCR analyses. We also purified the LcGal-3 protein that was expressed in prokaryotes, and found that it has the ability to agglutinate large yellow croaker red blood cells in a Ca²⁺-independent manner. The agglutination activity of LcGal-3 was inhibited by lipopolysaccharides (LPS) in a concentration-dependent manner, as shown in the sugar inhibition test. Additionally, LcGal-3 exhibited agglutination and antibacterial activities against three Gram-negative bacteria, including P. plecoglossicida, Vibrio parahaemolyticus, and Vibrio harveyi. Furthermore, we studied the agglutination mechanism of the LcGal-3 protein using blood coagulation tests with LcGal-3 deletion and point mutation proteins. Our results indicate that LcGal-3 protein plays a critical role in the innate immunity of the large yellow croaker, providing a basis for further studies on the immune mechanism and disease-resistant breeding in L. crocea and other marine fish.

Functional Analysis of a CTL-X-Type Lectin CTL16 in Development and Innate Immunity of Tribolium castaneum

C-type lectins (CTLs) are a class of proteins containing carbohydrate recognition domains (CRDs), which are characteristic modules that recognize various glycoconjugates and function primarily in immunity. CTLs have been reported to affect growth and development and positively regulate innate immunity in Tribolium castaneum. However, the regulatory mechanisms of TcCTL16 proteins are still unclear. Here, spatiotemporal analyses displayed that TcCTL16 was highly expressed in late pupae and early adults. TcCTL16 RNA interference in early larvae shortened their body length and narrowed their body width, leading to the death of 98% of the larvae in the pupal stage. Further analysis found that the expression level of muscle-regulation-related genes, including cut, vestigial, erect wing, apterous, and spalt major, and muscle-composition-related genes, including Myosin heavy chain and Myosin light chain, were obviously down-regulated after TcCTL16 silencing in T. castaneum. In addition, the transcription of TcCTL16 was mainly distributed in the hemolymph. TcCTL16 was significantly upregulated after challenges with lipopolysaccharides, peptidoglycans, Escherichia coli, and Staphylococcus aureus. Recombinant CRDs of TcCTL16 bind directly to the tested bacteria (except Bacillus subtilis); they also induce extensive bacterial agglutination in the presence of Ca²⁺. On the contrary, after TcCTL16 silencing in the late larval stage, T. castaneum were able to develop normally. Moreover, the transcript levels of seven antimicrobial peptide genes (attacin2, defensins1, defensins2, coleoptericin1, coleoptericin2, cecropins2, and cecropins3) and one transcription factor gene (relish) were significantly increased under E. coli challenge and led to an increased survival rate of T. castaneum when infected with S. aureus or E. coli, suggesting that TcCTL16 deficiency could be compensated for by increasing AMP expression via the IMD pathways in T. castaneum. In conclusion, this study found that TcCTL16 could be involved in developmental regulation in early larvae and compensate for the loss of CTL function by regulating the expression of AMPs in late larvae, thus laying a solid foundation for further studies on T. castaneum CTLs.

A Mannose Receptor from Litopenaeus vannamei Involved in Innate Immunity by Pathogen Recognition and Inflammation Regulation

Mannose receptor, as a member of the C-type lectin superfamily, is a non-canonical pattern recognition receptor that can internalize pathogen-associated ligands and activate intracellular signaling. Here, a mannose receptor gene, LvMR, was identified from the Pacific white shrimp Litopenaeus vannamei. LvMR encoded a signal peptide, a fibronectin type II (FN II) domain, and two carbohydrate-recognition domains (CRDs) with special EPS and FND motifs. LvMR transcripts were mainly detected in the hepatopancreas, and presented a time-dependent response after pathogen challenge. The recombinant LvMR (rLvMR) could bind to various PAMPs and agglutinate microorganisms in a Ca²⁺-dependent manner with strong binding ability to _D-mannose and N-acetyl sugars. The knockdown of LvMR enhanced the expression of most NF-κB pathway genes, inflammation and redox genes, while it had no obvious effect on the transcription of most phagocytosis genes. Moreover, the knockdown of LvMR caused an increase in reactive oxygen species (ROS) content and inducible nitric oxide synthase (iNOS) activity in the hepatopancreas after Vibrio parahaemolyticus infection. All these results indicate that LvMR might perform as a PRR in immune recognition and a negative regulator of inflammation during bacterial infection.

An LRR domain-containing membrane protein gene in rotifer Brachionus plicatilis: Sequence feature, expression pattern, and ligands binding activity

Leucine-rich repeat (LRR) domains mediate multiple innate immune responses via protein-ligand and protein-protein interactions, but their exact roles in invertebrates are poorly understood. Herein, an LRR domain-containing transmembrane protein (BpLRRm) was identified in the rotifer Brachionus plicatilis. The 1069 bp BpLRRm nucleotide sequence contains a 942 bp open reading frame (ORF) encoding a 313 amino acid polypeptide with four LRR motifs harbouring the LXXLXXLXLXXNXLXXL motif, and a transmembrane domain. Treatment with 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) decreased BpLRRm mRNA levels at 3 h, but they increased thereafter and peaked at 12 h. Lipopolysaccharide (LPS) treatment first increased BpLRRm mRNA levels at 3 h, but levels returned to normal at 12 h, then increased and peaked at 24 h. Recombinant BpLRRm protein bound pathogen-related molecular patterns (PAMPs), including LPS, peptidoglycan (PGN), glucan (GLU) and polyinosinic-polycytidylic acid (poly IC), in a dose-dependent manner. Thus, BpLRRm might function as a pattern recognition receptor (PRR) in the innate immunity of B. plicatilis, and mediate responses to environmental pollution.

Structural and evolutionary insights into the multidomain galectin from the red abalone Haliotis rufescens with specificity for sulfated glycans

Galectins are an evolutionarily ancient family of lectins characterized by their affinity for β-galactosides and a conserved binding site in the carbohydrate recognition domain (CRD). These lectins are involved in multiple physiological functions, including the recognition of glycans on the surface of viruses and bacteria. This feature supports their role in innate immune responses in marine mollusks. Here, we identified and characterized a galectin, from the mollusk Haliotis rufescens (named HrGal), with four CRDs that belong to the tandem-repeat type. HrGal was purified by affinity chromatography in a galactose-agarose resin and exhibited a molecular mass of 64.11 kDa determined by MALDI-TOF mass spectrometry. The identity of HrGal was verified by sequencing, confirming that it is a 555 amino acid protein with a mass of 63.86 kDa. This protein corresponds to a galectin reported in GenBank with accession number AHX26603. HrGal is stable in the presence of urea, reducing agents, and ions such as Cu²⁺ and Zn²⁺. The recombinant galectin (rHrGal) was purified from inclusion bodies in the presence of these ions. A theoretical model obtained with the AlphaFold server exhibits four non-identical CRDs, with a β sandwich folding and the representative motifs for binding β-galactosides. This allows us to classify HrGal within the tandem repeat galectin family. On the basis of a phylogenetic analysis, we found that the mollusk sequences form a monophyletic group of tetradomain galectins unrelated to vertebrate galectins. HrGal showed specificity for galactosides and glucosides but only the sulfated sugars heparin and ι-carrageenan inhibited its hemagglutinating activity with a minimum inhibitory concentration of 4 mM and 6.25 X 10^-5% respectively. The position of the sulfate groups seemed crucial for binding, both by carrageenans and heparin.

A novel c-type lysozyme from Litopenaeus vannamei exhibits potent antimicrobial activity

Lysozyme is relevant to the innate immune system as a vital protein for crustaceans. In the present study, we cloned and characterized a novel c-type lysozyme gene (LvLYZ) from the Pacific white shrimp (Litopenaeus vannamei). The obtained full-length cDNA of LvLYZ was 990 bp and contained an open reading frame of 693 bp. Its deduced amino acid sequence consisted of 230 amino acids (aa) with a 17 aa signal peptide at the N-terminal and 130 aa functional domains. The multiple sequence alignment (MSA) indicated that the typical active sites in LvLYZ were similarly conserved as c-type lysozymes from other species. The transcription of LvLYZ appeared in all detected tissues and had relatively higher expression levels in hemocytes, hepatopancreas, gill and intestine. The mRNA expression profiles of LvLYZ were up-regulated in hemocyte and hepatopancreas post the stimulation of Vibrio parahaemolyticus or white spot syndrome virus (WSSV), respectively. The recombinant protein of LvLYZ (rLvLYZ) exhibited antibacterial activities against various microbes, including Escherichia coli, Vibrio splendidus, Micrococcaus luteus, Vibrio parahaemolyticus and Staphylococcus aureus. These results indicated that LvLYZ could cope with bacteria in L. vannamei and may play a significant role in immune response against invading pathogens.

A preliminary report of exploration of the exosomal shuttle protein in marine invertebrate Chlamys farreri

Exosomes are extracellular vesicles secreted by diverse cell under normal or abnormal physiological conditions, which could carry a range of bioactive molecules and play significant roles in biological processes, such as intercellular communication and immune response. In the current study, a preliminary study was performed to investigate the exosomal shuttle protein in Chlamys farreri (designated as CfesPro) and to predict the potential function of exosomes in scallop innate immunity. The serum derived exosomes (designated as CfEVs) were obtained from lipopolysaccharide (LPS)-stimulated C. farreri and untreated ones. After confirmation and characterization by transmission electron microscopy (TEM), nano-HPLC-MS/MS spectrometry was performed on CfEVs using a label-free quantitative method. Totally 2481 exosomal shuttle proteins were identified in CfEVs proteomic data, which included many innate immune related proteins. GO and KOG functional annotation showed that CfesPro participated in cellular processes, metabolism reactions, signaling transductions, immune responses and so on. Moreover, 1421 proteins in CfesPro were enriched to 324 pathways by KEGG analysis, including several immune-related pathways, such as autophagy, apoptosis and lysosome pathway. Meanwhile, eight autophagy-related proteins were initially identified in CfesPro, indicating that CfEVs had a potential role with autophagy. All these findings showed that CfEVs were involved in C. farreri innate immune defenses. This research would enrich the protein database of marine exosomes and provide a basis for the exploration of immune defense systems in marine invertebrates.

A pattern recognition receptor ficolin from Portunus trituberculatus (Ptficolin) regulating immune defense and hemolymph coagulation

Ficolins, belonging to the fibrinogen-related protein superfamily, are important pattern recognition receptors in innate immunity. Here, a ficolin gene Ptficolin was characterized from the swimming crab Portunus trituberculatus. The completed cDNA sequence of Ptficolin encoded a signal peptide, a coiled-coil region and a fibrinogen-like domain but without the typical collagen region of vertebrate ficolins. Ptficolin showed higher expression in stomach and hepatopancreas, and presented a time-dependent response after pathogen challenge and injury stimulation. The recombinant Ptficolin (rPtficolin) could bind to various PAMPs and microorganisms, and agglutinate microorganisms and rabbit erythrocytes in a Ca²⁺-dependent manner, with strong binding ability to N-acetyl sugars. Meanwhile, rPtficolin promoted the hemocyte phagocytosis and clearance activity of Vibrio, while Ptficolin knockdown impaired the bacterial phagocytosis and clearance ability, suggesting the opsonin activity of Ptficolin. Knockdown of Ptficolin could downregulate the transcription of most complement-like genes and AMPs, but enhance the expression of most proPO system-related genes and key genes of Toll, IMD and JNK pathways. Moreover, knockdown of Ptficolin led to the increased hemolymph clotting time and the decreased expression of clotting-related genes. Our results indicate that Ptficolin could recognize and eliminate invading pathogens, and might be a prominent component in hemolymph coagulation of crab.

Galectin, another lectin from Fenneropenaeus merguiensis contributed in shrimp immune defense

Numerous lectins act as pattern recognition receptors (PRRs) in the innate immune system of invertebrates. Here, a galectin (FmGal) was isolated from hemocytes of Fenneropenaeus merguiensis. FmGal contained one open reading frame encoding a peptide of 338 amino acids. The primary sequence of FmGal comprised a carbohydrate recognition domain with a specific galactose binding site. The FmGal transcripts were found mostly in hemocytes of healthy shrimp. The expression of FmGal was up-regulated upon challenge with Vibrio parahaemolyticus and white spot syndrome virus (WSSV). Gene-silencing with FmGal double-stranded RNA resulted in extreme down-regulation of FmGal. Knockdown with a co-injection of pathogens reduced the survival rate of shrimp. Recombinant protein of FmGal (rFmGal) required Ca²⁺ to agglutinate pathogenic bacteria and exhibited sugar-specificity to galactose, lactose, lipopolysaccharide (LPS) and lipoteichoic acid (LTA). The ELISA-validated binding of rFmGal revealed higher affinity to LTA than LPS. rFmGal did not exhibit antibacterial activity but could enhance the phagocytosis and encapsulation of pathogenic invaders by hemocytes. Encapsulation was suppressed by galactose and lactose. Moreover, rFmGal also promoted the in vivo clearance of V. parahaemolyticus. FmGal, a galectin in F. merguiensis, participated in shrimp immunity, functioning as a PRR which might be involved in certain cellular responses.

Characterization and functional study of a chimera galectin from yellow drum Nibea albiflora

Galectins are protein that participates in a variety of immune responses in the process of pathogenic infections. In the present study, a chimera galectin gene was screened from the transcriptome database of Nibea albiflora, which was named as YdGal-3. The results of qRT-PCR showed that the mRNA transcripts of YdGal-3 were ubiquitously distributed in all the detected tissues. After infection with Vibrio harveyi, the expression of YdGal-3 in liver, spleen, and head kidney increased significantly. Immunohistochemistry showed that YdGal-3 protein was widely expressed in the head kidney. The purified YdGal-3 protein by prokaryotic expression agglutinated red blood cells. Sugar inhibition assay showed that the agglutinating activity of YdGal-3 protein was inhibited by different sugars including lactose, D-galactose, and lipopolysaccharide. In addition, we mutated YdGal-3 His 294 into proline (P), alanine (A), glycine (G), and aspartic acid (D), it was further proved that the residue plays a key role in agglutination. YdGal-3 agglutinated some gram-negative bacteria including Pseudomonas plecoglossicida, Vibrio parahemolyticus, V. harveyi, and Aeromonas hydrophila, and exhibited antibacterial activity. These results suggested that YdGal-3 protein played an important role in the innate immunity of N. albiflora.

A C-Type Lectin Highly Expressed in Portunus trituberculatus Intestine Functions in AMP Regulation and Prophenoloxidase Activation

A C-type lectin (PtCLec2) from Portunus trituberculatus was identified for characterization of its role in defense and innate immunity. PtCLec2 contains a single carbohydrate-recognition domain (CRD) with a conserved QPD motif, which was predicted to have galactose specificity. The mRNA expression of PtCLec2 was predominantly detected in intestine and increased rapidly and significantly upon pathogen challenge. The recombinant PtCLec2 (rPtCLec2) could bind various microorganisms and PAMPs with weak binding ability to yeast and PGN. It agglutinated the tested Gram-negative bacteria (Vibrio alginolyticus and Pseudomonas aeruginosa), Gram-positive bacteria (Staphylococcus aureus and Micrococcus luteus), and rabbit erythrocytes in the presence of exogenous Ca²⁺, and these agglutination activities were suppressed by LPS, d-galactose, and d-mannose. Further, rPtCLec2 enhanced phagocytosis and clearance of V. alginolyticus, and displayed inhibitory activities against the tested bacteria. Knockdown of PtCLec2 decreased the transcription of two phagocytosis genes (PtArp and PtMyosin), three prophenoloxidase (proPO) system-related genes (PtPPAF, PtcSP1, and PtproPO), six antimicrobial peptides (AMPs) (PtALF4-7, PtCrustin1, and PtCrustin3), and PtRelish but upregulated the expression levels of PtJNK, PtPelle, and PtTLR. These results collectively indicate that PtCLec2 might perform its immune recognition function via binding and agglutination, and mediate pathogen elimination via regulating hemocyte phagocytosis, AMP synthesis, and proPO activation.

Proteomic analysis of hemocyte reveals the immune regulatory mechanisms after the injection of corticosteroid-releasing hormone in mud crab Scylla Paramamosain

Corticosteroid-releasing hormone (CRH) is a crucial neuroendocrine-immune factor regulating the immune response of Scylla paramamosain. To understand the regulatory mechanisms of CRH in S. paramamosain, the hemolymph of S. paramamosain with injection of CRH (1.5 ng/crab) at 24 h were chosen to perform proteomic analysis in this study. Furthermore, quantitative real-time PCR (RT-PCR) method was used to validate the accuracy of proteomic data at 24 h after CRH injection. The proteomic data showed that 255 DEPs were identified, in which 231 and 24 were up- or down-regulated, respectively. Besides, the results of enriched pathways showed that the DEPs were involved in signaling pathways, cellular immunity, humoral immunity and the response of immune related processes. These results revealed that CRH promoted the activation of signal transduction, regulated immune systems and antioxidation, and enhanced the immune related processes (such as protein synthesis, protein transport, carbohydrate mobilization and energy redistribution). These findings will benefit to foster the understanding on the effects of glucocorticoids on neuroendocrine-immune (NEI) networks of crustacean, and supply a substantial material and foundation for further researching of the NEI response. SIGNIFICANCE: Corticotrophin-releasing hormone (CRH) is a 41-amino acid neuropeptide and has been preliminarily studied in aquatic animals. CRH can regulate many important physiological activities comprising protein synthesis, energy metabolism, growth, breeding and behavior in fish, which play an important roles in neuroendocrine-immune (NEI) regulatory network of fish. The neuroendocrine system of crustacean has a primary research, that inspired by fish NEI network. Despite the research on the neuroendocrine system in crustacean has rapidly increased in recent years, our understanding of the regulation between neuroendocrine system and immune system in crustacean is still limited. The research on the strategy of NEI network in crustaceans becomes a significant issue. In the present study, the isobaric tags for relative and absolute quantification (iTRAQ) technology approach were applied to examine the NEI network of Scylla Paramamosain. control group and treatment group (CRH: 1.5 ng/crab) were settled for the iTRAQ experiment, and sampled at 24 h after CRH injection. The study aimed to gain knowledge on the immune response in Scylla Paramamosain after CRH injection and identify related differentially expressed proteins (DEPs) of the crab. The results of this study provide a preliminary resource for analysis the immune mechanism for crustaceans. In general, our work represents the first report of the utilization of the iTRAQ proteomics method for the study of NEI regulatory network in Scylla Paramamosain after CRH injection. We identified a number of DEPs involved in diverse pathways including immune signaling pathways, cellular immunity, humoral immunity, immune related process. These results demonstrated a very complex network involving immune and multiple related metabolic pathways in hemocytes of Scylla Paramamosain and will be of great value in understanding the crab neuroendocrine-immune immune mechanism.

A DM9-containing protein from oyster Crassostrea gigas (CgDM9CP-3) mediating immune recognition and encapsulation

DM9 domain containing protein (DM9CP) is a recently identified pattern recognition molecules exiting in most organisms except plants. In the present study, a novel DM9-containing protein (CgDM9CP-3) was identified from Pacific oyster Crassostrea gigas with an open reading frame of 438 bp, encoding a polypeptide of 145 amino acids containing two tandem DM9 repeats. The deduced amino acid sequence of CgDM9CP-3 shared 52.4% and 58.6% identity with CgDM9CP-1 and CgDM9CP-2, respectively. The mRNA transcripts of CgDM9CP-3 were highest expressed in oyster gills and its protein was mainly distributed in cytomembrane of haemocytes. After the stimulations with Vibrio splendidus and mannose, the mRNA expression of CgDM9CP-3 in oyster gills was significantly up-regulated and reached the peak level at 12 h and 24 h (p < 0.05), which was 7.80-fold (p < 0.05) and 42.82-fold (p < 0.05) of that in the control group, respectively. The recombinant CgDM9CP-3 protein (rCgDM9CP-3) was able to bind LPS, PGN and d-Mannose, fungi Pichia pastoris and Yarrowia lipolytica, as well as gram-negative bacteria Escherichia coli, Vibrio anguillarum and V. splendidus in a Ca²⁺-dependent manner. Moreover, it could enhance the encapsulation of haemocytes and exhibited agglutination activity towards fungi P. pastoris and Y. lipolytica in vitro with Ca²⁺. These results suggested that CgDM9CP-3 not only acted as a PRR involved in the pathogen recognition, but also enhanced cellular encapsulation in oyster C. gigas.

Immune Responses to Gram-Negative Bacteria in Hemolymph of the Chinese Horseshoe Crab, Tachypleus tridentatus

Chinese horseshoe crab, Tachypleus tridentatus, is an ancient marine arthropod with a long evolutionary history. As a kind of living fossil species, the pathogen defenses of horseshoe crabs entirely depend on the innate immune system. Although, there are abundant immune molecules found in the horseshoe crab hemolymph, the biological mechanisms underlying their abilities of distinguishing and defending against invading microbes are still unclear. In this study, we used high-throughput sequencing at mRNA and protein levels and bioinformatics analysis methods to systematically analyze the innate immune response to Gram-negative bacteria in hemolymph of Chinese horseshoe crab. These results showed that many genes in the complement and coagulation cascades, Toll, NF-κB, C-type lectin receptor, JAK-STAT, and MAPK signaling pathways, and antimicrobial substances were activated at 12 and 24 h post-infection, suggesting that Gram-negative bacteria could activate the hemolymph coagulation cascade and antibacterial substances release via the above pathways. In addition, we conjectured that Toll and NF-κB signaling pathway were most likely to participate in the immune response to Gram-negative bacteria in hemolymph of horseshoe crab through an integral signal cascade. These findings will provide a useful reference for exploring the ancient original innate immune mechanism.

A C-type lectin with dual-CRD from Tribolium castaneum is induced in response to bacterial challenge

BACKGROUND

C-type lectins (CTLs), a group of pattern recognition receptors, are involved in regulating the immune response of insects and could be used as potential targets for pest control. However, information about roles of CTLs in the innate immunity of Tribolium castaneum, a serious, worldwide pest that damages stored grain products, is relatively scarce.

RESULTS

Here, a CTL with dual carbohydrate recognition domains (CRDs) containing a highly conserved WHD (Trp⁵³ -His⁵⁴ -Asp⁵⁵ ) motif was identified in T. castaneum and named as TcCTL3. Spatiotemporal analysis showed that TcCTL3 was highly expressed in all developmental stages except early eggs, and mainly distributed in central nervous system and hemolymph. The transcript levels of TcCTL3 were significantly increased after lipopolysaccharide (LPS) and peptidoglycan (PGN) stimulation. Recombinant TcCTL3 was able to bind directly to LPS, PGN and all tested bacteria and induce a broad spectrum of microbial agglutination in the presence of Ca²⁺ . The binding was shown mainly through CRD1 domain of TcCTL3. When TcCTL3 was knocked down by RNA interference, expression of nine antimicrobial peptides (AMPs) (attacin1, attacin2, attacin3, defensins1, defensins2, coleoptericin1, coleoptericin2, cecropins2 and cecropins3) and four transcription factors (TFs) (dif1, dif2, relish and jnk) were significantly decreased under LPS and PGN stimulation, leading to increased mortality of T. castaneum when infected with Gram-positive Staphylococcus aureus or Gram-negative Escherichia coli infection.

CONCLUSION

TcCTL3 could mediate the immune response in T. castaneum via the pattern recognition, agglutination and AMP expression. These findings indicate a potential mechanism of TcCTL3 in resisting bacteria and provide an alternative molecular target for pest control. © 2020 Society of Chemical Industry.

A typical C-type lectin, perlucin-like protein, is involved in the innate immune defense of whiteleg shrimp Litopenaeus vannamei

C-type lectins are a large group of the pattern-recognition proteins, and have been reported to be involved in invertebrate innate immunity, such as cell adhesion, bacterial clearance, phagocytosis, prophenoloxidase activation and encapsulation. Here, a perlucin-like protein (PLP), a typical C-type lectin, was identified from the cDNA library of the shrimp, Litopenaeus vannamei. LvPLP contains a 540 bp open reading frame, encoding a protein of 179 amino acids that includes a single carbohydrate-recognition domain. Phylogenetic analysis showed that LvPLP was clustered into a single group together with other perlucins from molluscs. Quantitative real-time PCR revealed that LvPLP was expressed mainly in the hemocytes, hemolymph, heart and gills. The transcription of LvPLP was significantly induced at 9 h by both Gram^- bacteria Vibrio parahaemolyticus and Vibrio anguillarum. Meanwhile, recombinant LvPLP (rLvPLP) bound directly to lipopolysaccharide and peptidoglycan with different affinity. rLvPLP showed a strong ability to bind to Gram⁺ (Staphylococcus aureus and Bacillus subtilis) and Gram^- bacteria (V. parahaemolyticus and V. anguillarum), and could induce agglutination of V. parahaemolyticus and V. anguillarum, but not S. aureus and B. subtilis in the presence Ca²⁺. Further study showed that when LvPLP was knocked down by RNAi, three phagocytosis-related genes (peroxinectin, mas-like protein and dynamin) and four antimicrobial peptide (AMP) genes (crustin, ALF1, ALF2 and ALF3) were significantly decreased. Altogether, these results demonstrated that LvPLP played a vital role in L. vannamei immune response towards bacterial challenge by binding and agglutinating bacteria and influencing phagocytosis and AMP expression.

A novel C-type lectin with a YPD motif from Portunus trituberculatus (PtCLec1) mediating pathogen recognition and opsonization

C-type lectins are a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins that function as pattern recognition receptors (PRRs) in innate immune system. In this study, a new C-type lectin was identified from the swimming crab Portunus trituberculatus (PtCLec1). The full-length cDNA of PtCLec1 was 873 bp encoding 176 amino acids. The predicted PtCLec1 protein contained a signal peptide and a single carbohydrate-recognition domain with a special YPD motif. The PtCLec1 transcripts were mainly detected in hepatopancreas and its relative expression levels were significantly up-regulated after the challenges of Vibrio alginolyticus, Micrococcus luteus and Pichia pastoris. The recombinant PtCLec1 (rPtCLec1) could bind all the tested pathogen-associated molecular patterns (PAMPs), including lipopolysaccharides (LPS), peptidoglycan (PGN) and glucan (GLU), and microorganisms, including V. alginolyticus, V. parahaemolyticus, Pseudomonas aeruginosa, Staphylococcus aureus, M. luteus and P. pastoris. It also exhibited strong activity to agglutinate bacteria and yeast in a Ca²⁺-dependent manner, and such agglutinating activity could be inhibited by d-galactose and LPS. Moreover, rPtCLec1 revealed antimicrobial activity against the tested Gram-negative (V. alginolyticus, V. parahaemolyticus and P. aeruginosa) and Gram-positive bacteria (S. aureus and M. luteus), and promoted the clearance of V. alginolyticus in vivo and hemocyte phagocytosis in vitro. Knockdown of PtCLec1 could down-regulate the expression of phagocytosis-related genes, but enhance the expression levels of prophenoloxidase (proPO) system-related genes, mannose-binding lectin (MBL), antimicrobial peptides (AMPs), MyD88 and Relish. All these results indicate that PtCLec1 might act as a PRR in immune recognition and an opsonin in pathogen elimination.

A tandem-repeat galectin-1 from Apostichopus japonicus with broad PAMP recognition pattern and antibacterial activity

Galectins belong to the family of carbohydrate-binding proteins and play major roles in the immune and inflammatory responses of both vertebrates and invertebrates. In the present study, one novel galectin-1 protein named AjGal-1 was identified from Apostichopus japonicas with an open reading frame of 1179 bp encoding a polypeptide of 392 amino acids. The deduced amino acids sequence of AjGal-1 contained three carbohydrate recognition domains (CRDs) which shared 34-37% identity with that of other galectin proteins from echinodermata, fishes, and birds. In the phylogenetic tree, AjGal-1 was closely clustered with galectins from Mesocentrotus nudus and Paracentrotus lividus. The mRNA transcripts of AjGal-1 were ubiquitously expressed in all the detected tissues, including gut, longitudinal muscle, gonad, coelomocytes, respiratory tree, tentacle and body wall, with the highest expression level in coelomocytes. After Vibrio splendidus stimulation, the mRNA expression levels of AjGal-1 in coelomocytes were significantly increased at 6 and 12 h (P < 0.01) compared with that in control group, and went back to normal level at 72 h. The recombinant protein of AjGal-1 (rAjGal-1) could bind various PAMPs including d-galactose, lipopolysaccharide (LPS), peptidoglycan (PGN) and mannose (Man), and exhibited the highest affinity to d-galactose. Meanwhile, rAjGal-1 could also bind and agglutinate different kinds of microorganisms, including gram-negative bacteria (V. splendidus and Escherichia coli), gram-positive bacteria (Micrococus leteus), and fungi (Pichia pastoris). rAjGal-1 also exhibited anti-microbial activity against V. splendidus and E. coli. All these results suggested that AjGal-1 could function as an important PRR with broad spectrum of microbial recognition and anti-microbial activity against the invading pathogen in A. japonicas.

Identification and characterization of a novel galectin from the mud crab Scylla paramamosain

Galectins are a family of β-galactoside-binding lectins that play key roles in the invertebrate innate immunity system, but no galectin genes have been identified in the mud crab (Scylla paramamosain) so far. The present study is the first to clone a galectin gene (SpGal) from S. paramamosain, by the rapid amplification of cDNA ends technique based on expressed sequence tags. The full-length cDNA of SpGal was 3142 bp. Its open reading frame encoded a polypeptide of 280 amino acids containing a GLECT/Gal-bind lectin domain and a potential N-glycosylation site. The deduced amino acid sequence and multi-domain organization of SpGal were highly similar to those of invertebrate galectins, and phylogenetic analysis showed that SpGal was closely related to galectin isolated from Portunus trituberculatus. The mRNA transcripts of SpGal were found to be constitutively expressed in a wide range of tissues, with its expression level being higher in the hepatopancreas, gill, and hemocytes. The mRNA expression level of SpGal increased rapidly after the crabs were stimulated by Vibrio alginolyticus, and the maximum expression appeared at 6 h after the challenge. The lipopolysaccharide-binding ability of SpGal was dependent on its concentration, and it also exhibited agglutination activity with three Gram-negative (Aeromonas hydrophila, Chryseobacterium indologenes and Vibrio alginolyticus) and three Gram-positive (Bacillus aquimaris, Staphylococcus aureus and Micrococcus lysodeik) bacterial strains. In addition, hemagglutination activity with rabbit erythrocytes was observed in the absence of d-galactose. These results indicate that SpGal in S. paramamosain acts as a pattern recognition receptor to recognize a broad spectrum of microbes. The findings together indicate that SpGal plays an important role in the innate immune mechanisms of S. paramamosain against pathogenic infection.

Research progress in innate immunity of freshwater crustaceans

Invertebrates lack adaptive immunity and innate immunity plays important roles in combating foreign invasive pathogens. Freshwater crustaceans, which are invertebrates, depend completely on their innate immune system. In recent years, many immune-related molecules in freshwater crustaceans, as well as their functions, have been identified. Three main immune signaling pathways, namely, Toll, immune deficiency (IMD), and Janus kinase-signal transducer activator of transcription (JAK/STAT) pathways, were found in freshwater crustaceans. A series of pattern recognition receptors (PRRs), including Toll receptors, lectins, lipopolysaccharide and β-1,3-glucan binding protein, scavenger receptors, Down syndrome cell adhesion molecules, and thioester-containing proteins, were reported. Prophenoloxidase activation system and antimicrobial peptide synthesis are two important immune effector systems. These components are involved in the innate immunity of freshwater crustaceans, and they function in the innate immune defense against invading pathogens. This review mainly summarizes innate immune signaling pathways, PRRs, and effector molecules in freshwater crustaceans.

Adaptation evolution and bioactivity of galectin from the deep sea Vesicomyidae clam Archivesica packardana

Hydrothermal vents and cold seep zones are two special habitats in the deep sea. These habitats are always dark, and have extreme temperatures (low or high), heavy metals and toxic substances (sulfide, methane). Vesicomyidae clams, which maintain endosymbionts in their gills, are common species in these two special zones and are thought to develop an efficacious immune system against unusual habitats. In the present study, a novel galectin (Apgalectin) was identified from the Vesicomyidae clam Archivesica packardana. The phylogenetic tree indicated that Apgalectin had two CRDs and was closely clustered with galectins from invertebrates, especially mollusks. A branch-site model showed that 9 positively selected sites (ω2 = 6.83950) were identified comparing to galectins from the Order Veneroida, implying a different function of Vesicomyidae galectins. A microbe binding assay showed that rApgalectin could bind to gram-positive bacteria, gram-negative bacteria and fungi. A PAMP binding assay indicated that Apgalectin could bind LPS, PGN, β-1,3-glucan, glucan from yeast and Poly I:C in dose-dependent manner. Apgalectin only agglutinated Micrococcus luteus and agglutination could be inhibited by galactose which demonstrated that Apgalectin might be involved in immune defense by recognizing and binding bacteria in a β-galactoside manner. Further experiments showed that Apgalectin might play an indirect effector role in the immune response because of its limited antibacterial spectrum. All analyses validated that Apgalectin from Archivesica packardana plays a variety of functions in immune responses and provided basal information for the immune study of deep-sea mollusks.

Natural and synthetic pathogen associated molecular patterns modulate galectin expression in cow blood

Pathogen-associated Molecular Patterns (PAMPs) are highly conserved structural motifs that are recognized by Pathogen Recognition receptors (PRRs) to initiate immune responses. Infection by these pathogens and the immune response to PAMPS such as lipopolysaccharide (LPS), Peptidoglycan (PGN), bacterial oligodeoxynucleotides [CpG oligodeoxynucleotides 2006 (CpG ODN2006) and CpG oligodeoxynucleotides 2216 (CpG ODN2216)], and viral RNA Polyinosinic-Polycytidylic Acid (Poly I:C), are associated with infectious and metabolic diseases in animals impacting health and production. It is established that PAMPs mediate the production of cytokines by binding to PRRs such as Toll-like receptors (TLR) on immune cells. Galectins (Gal) are carbohydrate-binding proteins that when expressed play essential roles in the resolution of infectious and metabolic diseases. Thus it is important to determine if the expression of galectin gene (LGALS) and Gal secretion in blood are affected by exposure to LPS and PGN, PolyI:C and bacterial CpG ODNs. LPS increased transcription of LGALS4 and 12 (2.5 and 2.02 folds respectively) and decreased secretion of Gal 4 (p < 0.05). PGN increased transcription of LGALS-1, -2, -3, -4, -7, and -12 (3.0, 2.3, 2.0, 4.1, 3.3, and 2.4 folds respectively) and secretion of Gal-8 and Gal-9 (p < 0.05). Poly I:C tended to increase the transcription of LGALS1, LGALS4, and LGALS8 (1.78, 1.88, and 1.73 folds respectively). Secretion of Gal-1, -3, -8 and nine were significantly increased in treated samples compared to control (p < 0.05). CpG ODN2006 did not cause any significant fold changes in LGALS transcription (FC < 2) but increased secretion of Gal-1, and-3 (p < 0.05) in plasma compared to control. Gal-4 was however reduced in plasma (p < 0.05). CpG ODN2216 increased transcription of LGALS1 and LGALS3 (3.8 and 1.6 folds respectively), but reduced LGALS2, LGALS4, LGALS7, and LGALS12 (–1.9, –2.0, –2.0 and; –2.7 folds respectively). Secretion of Gal-2 and -3 in plasma was increased compared to control (p < 0.05). Gal-4 secretion was reduced in plasma (p < 0.05). The results demonstrate that PAMPs differentially modulate galectin transcription and translation of galectins in cow blood.

Characterization and functional analysis of a novel mannose-binding lectin from the swimming crab Portunus trituberculatus

Mannose-binding lectin (MBL) is a pattern recognition receptor (PRR) that plays an important role in the innate immune response. In this study, a novel mannose-binding lectin was cloned from the swimmimg crab Portunus trituberculatus (designated as PtMBL). The complete cDNA of PtMBL gene was 1208 bp in length with an open reading frame (ORF) of 732 bp that encoded 244 amino acid proteins. PtMBL shared lower amino acid similarity with other MBLs, yet it contained the conserved carbohydrate-recognition domain (CRD) with QPD motif and was clearly member of the collectin family. PtMBL transcripts were mainly detected in eyestalk and gill with sexually dimorphic expression. The temporal expression of PtMBL in hemocytes showed different activation times after challenged with Vibrio alginolyticus, Micrococcus luteus and Pichia pastoris. The recombinant PtMBL protein revealed antimicrobial activity against the tested Gram-negative and Gram-positive bacteria. It could also bind and agglutinate (Ca²⁺-dependent) both bacteria and yeast. Furthermore, the agglutinating activity could be inhibited by both d-galactose and d-mannose, suggesting the broader pathogen-associated molecular patterns (PAMPs) recognition spectrum of PtMBL. These results together indicate that PtMBL could serve as not only a PRR in immune recognition but also a potential antibacterial protein in the innate immune response of crab.

Identification and characterisation of a novel small galectin in razor clam (Sinonovacula constricta) with multiple innate immune functions

Galectins are lectins possessing an evolutionarily conserved carbohydrate recognition domain (CRD) with affinity for β-galactoside. The key role played by innate immunity in invertebrates has recently become apparent. Herein, a full-length galectin (ScGal) was identified in razor clam (Sinonovacula constricta). The 528 bp open reading frame encodes a polypeptide of 176 amino acids with a single CRD and no signal peptide. ScGal mRNA transcripts were mainly expressed in hemolymph and gill, and were significantly up-regulated following bacterial challenge. Recombinant rScGal protein binds to and aggregates various bacteria, and has affinity for peptidoglycan, lipoteichoic acid and _d-galactose. The protein also stimulates hemocytes to phagocytose invading bacterial pathogens. ScGal is an important immune factor in innate immunity, and a small protein with multiple important functions.

Galactoside-binding lectin in Solen grandis as a pattern recognition receptor mediating opsonization

Galactoside-binding lectin (galectin) is a type of pathogen recognition molecule that occupies an important position in the invertebrate innate immunity system. Our previous study has identified a galectin gene in mollusk Solen grandis (SgGal-1) and illustrated its potential roles in innate immunity. By the functional study using recombinant protein and specific antibody, here, we confirmed the pivotal roles of SgGal-1 in immune defense of S. grandis. SgGal-1 protein was expressed in many tested tissues including gill, mantle, hepatopancreas and gonad, except hemocytes and muscle. The recombinant SgGal-1 (rSgGal-1) bound PGN and β-glucan instead of LPS in vitro, and it further caused significant agglutination of five different microbes, suggesting SgGal-1 served as a pattern recognition receptor (PRR) involved in immune defense of mollusk. Furthermore, SgGal-1 recruited hemocytes to encapsulate, which was blocked by anti-rSgGal-1 serum. In the meantime, rSgGal-1 as well as promoted the phagocytosis of hemocytes against Escherichia coli in vitro. All these results suggested that SgGal-1 in S. grandis not only acted as a PRR recognizing microbes but also directly participated in the process of immune opsonization to protect the host from pathogenic infection.

The cyclin-dependent kinase 2 (CDK2) mediates hematopoiesis through G1-to-S transition in Chinese mitten crab Eriocheir sinensis

Cyclin-dependent kinases (CDKs), a family of cell cycle-related serine/threonine kinases, participate in various biological processes, and play crucial roles in the innate immunity. In the present study, a CDK2 (designed as EsCDK2) with a serine/threonine protein kinase catalytic domain was identified from Chinese mitten crab (Eriocheir sinensis). The full-length cDNA sequence of EsCDK2 was of 2405 bp with an open reading frame (ORF) of 909 bp. EsCDK2 shared 66%-81% sequence similarities with previously identified CDK2s. It was clustered with the CDK2 from Penaeus monodon in the invertebrate branch of the phylogenetic tree. The mRNA transcripts of EsCDK2 were highly expressed in hematopoietic tissue (HPT) and gonad, while lower in hemocytes, heart, gills, and muscle. EsCDK2 protein distributed in both cytoplasm and nucleus of HPT cells. The expression of EsCDK2 mRNA in HPT was significantly up-regulated and peaked at 3 h post stimulations with Aeromonas hydrophila (2.31-fold, p < 0.05) and Lipopolysaccharide (LPS) (2.02-fold, p < 0.05). After exsanguination, the total hemocyte counts (THC) decreased significantly to 0.42 × 10⁷/ml (0.39-fold, p < 0.05) at 0.5 h, then returned to a normal level at 6 h, while the mRNA expression of EsCDK2 in HPT cells was up-regulated at the early phase from 0.5 h to 6 h. After injection of EsCDK2-dsRNA, the mRNA expression level of EsCDK2 in HPT and THC both decreased to 0.53-fold (p < 0.01) and 0.78-fold (p < 0.05) at 24 h, respectively, and the percentage of new-born hemocytes in HPT also decreased significantly from 37.7% to 16.3% (0.43-fold, p < 0.01). After knocking down of EsCDK2, THC decreased dramatically at 6 h (0.65-fold, p < 0.01) post exsanguination, while returned normal at 6 h in PBS group. After interference of EsCDK2 mRNA expression, the percentage of G0-G1 phase cells significantly increased to 85.01% (1.26-fold, p < 0.01), while S phase and G2-M phase cells significantly decreased to 7.92% (0.46-fold, p < 0.01) and 7.07% (0.43-fold, p < 0.01) respectively, indicating that the cell cycle of HPT cells arrested at G1 phase. These results collectively demonstrated that EsCDK2 participated in the regeneration of hemocytes or hematopoiesis by regulating the transition from G1 to S phase in the cell cycle, and involves in the innate immune responses of E. sinensis.

A galectin contributes to the innate immune recognition and elimination of pathogens in the freshwater mussel Hyriopsis cumingii

Galectins are members of the lectin superfamily. They function as pattern recognition receptors in the innate immune system of vertebrates and invertebrates. A galectin homolog from the triangle sail mussel Hyriopsis cumingii (HcGal2) was cloned and characterized. HcGal2 mRNA was expressed in all tissues examined, displaying particular enrichment in mantle tissue. Interestingly, rHcGAL2 protein was only detected in the mantle, hemocytes, and gills, suggesting that post-transcriptional regulation may occur. HcGal2 expression was induced in the mantle, liver, and hemocytes after exposure to lipopolysaccharides, Gram-negative bacteria (Aeromonas hydrophila), and Gram-positive bacteria (Staphylococcus aureus). The transcript significant upregulated was also detected after implantation in the mantle, pearl sac, liver, and hemocytes. Recombinant HcGAL2 protein (rHcGAL2) agglutinated Gram-positive and Gram-negative bacteria. In addition, rHcGAL2 promoted phagocytosis by hemocytes in vivo. Our data suggest that HcGal2 functioned as a pattern recognition receptor in against the pathogenic microbes and contributed to the "non-self" recognition and elimination in H. cumingii.

Glycogen synthase kinase-3 (GSK3) regulates TNF production and haemocyte phagocytosis in the immune response of Chinese mitten crab Eriocheir sinensis

Glycogen synthase kinase-3 (GSK3) is a serine/threonine protein kinase firstly identified as a regulator of glycogen synthesis. Recently, it has been proved to be a key regulator of the immune reaction. In the present study, a GSK3 homolog gene (designated as EsGSK3) was cloned from Chinese mitten crab, Eriocheir sinensis. The open reading frame (ORF) was 1824 bp, which encoded a predicted polypeptide of 607 amino acids. There was a conserved Serine/Threonine Kinase domain and a DNA binding domain found in EsGSK3. Phylogenetic analysis showed that EsGSK3 was firstly clustered with GSK3-β from oriental river prawn Macrobrachium nipponense in the invertebrate branch, while GSK3s from vertebrates formed the other distinct branch. EsGSK3 mRNA transcripts could be detected in all tested tissues of the crab including haepatopancreas, eyestalk, muscle, gonad, haemocytes and haematopoietic tissue with the highest expression level in haepatopancreas. And EsGSK3 protein was mostly detected in the cytoplasm of haemocyte by immunofluorescence analysis. The expression levels of EsGSK3 mRNA increased significantly at 6 h after Aeromonas hydrophila challenge (p < 0.05) in comparison with control group, and then gradually decreased to the initial level at 48 h (p > 0.05). The mRNA expression of lipopolysaccharide-induced tumor necrosis factor (TNF)-α factor (EsLITAF) was also induced by A. hydrophila challenge. However, the mRNA expression of EsLITAF and TNF-α production was significantly suppressed after EsGSK3 was blocked in vivo with specific inhibitor lithium, while the phagocytosis of crab haemocytes was significantly promoted. These results collectively demonstrated that EsGSK3 could regulate the innate immune responses of E. sinensis by promoting TNF-α production and inhibiting haemocyte phagocytosis.

High Bacterial Agglutination Activity in a Single-CRD C-Type Lectin from Spodoptera exigua (Lepidoptera: Noctuidae)

Lectins are carbohydrate-interacting proteins that play a pivotal role in multiple physiological and developmental aspects of all organisms. They can specifically interact with different bacterial and viral pathogens through carbohydrate-recognition domains (CRD). In addition, lectins are also of biotechnological interest because of their potential use as biosensors for capturing and identifying bacterial species. In this work, three C-type lectins from the Lepidoptera Spodoptera exigua were produced as recombinant proteins and their bacterial agglutination properties were characterized. The lowest protein concentration producing bacterial agglutination against a panel of different Gram+ and Gram− as well as their carbohydrate binding specificities was determined for the three lectins. One of these lectins, BLL2, was able to agglutinate cells from a broad range of bacterial species at an extremely low concentration, becoming a very interesting protein to be used as a biosensor or for other biotechnological applications involving bacterial capture.

A galectin from Hyriopsis cumingii involved in the innate immune response against to pathogenic microorganism and its expression profiling during pearl sac formation

Hyriopsis cumingii is the most important freshwater pearl mussel cultured in China. The operation for implantation is one necessary technical step for pearl culture. However, implantation-induced trauma results in a series of immune responses and can enable the invasion of pathogenic microbes. Lectin proteins are found widely in nature and play important roles in innate immunity. Galectins are members of the lectin superfamily and are characterized by one or several carbohydrate recognition domains (CRDs) that produce multiple sugar binding sites on the protein. Here we cloned and characterized the H. cumingii galectin gene HcGal1, which encodes a 312 amino acid galectin protein. The HcGal1 transcript was detected in all tested H. cumingii tissues and showed higher expression specifically in immune tissues. The significant upregulation of HcGal1 expression was observed after challenging the mussel with lipopolysaccharide or Gram-negative and Gram-positive bacteria. After implantation, significant downregulation of the HcGal1 transcript was noted in the mantle, hemocytes, and pearl sac in the acute-stress stage (0-24 h) and the stage of wound healing and pearl-sac formation (24 h-7 d). In addition, significant upregulation of HcGal1 expression was observed in the liver in the stage of wound healing and pearl-sac formation. In the pearl-secretion stage (7-35 d), the HcGal1 transcript levels returned to normal in all tested tissues. We also show that recombinantly expressed and purified HcGal1 can agglutinate some Gram-negative and Gram-positive bacteria. In addition, in vivo experiments showed that the recombinant protein HcGal1 could promote phagocytosis by hemocytes. Our data suggest that HcGal1 plays a role in innate immune responses involved in pathogen recognition and wound healing.