The functional characterization and comparison of two single CRD containing C-type lectins with novel and typical key motifs from Portunus trituberculatus

Comparative transcriptomics reveals the immune dynamics during the molting cycle of swimming crab Portunus trituberculatus

Molting is one of the most important biological processes of crustacean species, and a number of molecular mechanisms facilitate this complex procedure. However, the understanding of the immune mechanisms underlying crustacean molting cycle remains very limited. This study performed transcriptome sequencing in hemolymph and hepatopancreas of the swimming crab (Portunus trituberculatus) during the four molting stages: post-molt (AB), inter-molt (C), pre-molt (D), and ecdysis (E). The results showed that there were 78,572 unigenes that were obtained in the hemolymph and hepatopancreas of P. trituberculatus. Further analysis showed that 98 DEGs were involved in immunity response of hemolymph and hepatopancreas, and most of the DEGs participated in the process of signal transduction, pattern recognition proteins/receptors, and antioxidative enzymes system. Specifically, the key genes and pathway involved in signal transduction including the GPCR126, beta-integrin, integrin, three genes in mitogen-activated protein kinase (MAPK) signaling cascade (MAPKKK10, MAPKK4, and p38 MAPK), and four genes in Toll pathway (Toll-like receptor, cactus, pelle-like kinase, and NFIL3). For the pattern recognition proteins/receptors, the lowest expression level of 11 genes was found in the E stage, including C-type lectin receptor, C-type lectin domain family 6 member A and SRB3/C in the hemolymph, and hepatopancreatic lectin 4, C-type lectin, SRB, Down syndrome cell adhesion molecule homolog, Down syndrome cell adhesion molecule isoform, and A2M. Moreover, the expression level of copper/zinc superoxide dismutase isoform 4, glutathione peroxidase, glutathione S-transferase, peroxiredoxin, peroxiredoxin 6, and dual oxidase 2 in stage C or stage D significantly higher than that of stage E or stage AB. These results fill in the gap of the continuous transcriptional changes that are evident during the molting cycle of crab and further provided valuable information for elucidating the molecular mechanisms of immune regulation during the molting cycle of crab.

Transcriptomic analysis of male three-spot swimming crab (Portunus sanguinolentus) infected with the parasitic barnacle Diplothylacus sinensis

Diplothylacus sinensis is reported as an intriguing parasitic barnacle that can negatively affect the growth, molting, reproduction in several commercially important portunid crabs. To better understand the molecular mechanisms of host-parasite interactions, we characterized the gene expression profiles from the healthy and D. sinensis infected Portunus sanguinolentus by high-through sequence method. Totally, the transcriptomic analysis generated 52, 266, 600 and 51, 629, 604 high quality reads from the infected and control groups, respectively. The clean reads were assembled to 90,740 and 69,314 unigenes, with the average length of 760 bp and 709 bp, respectively. The expression analysis showed that 18,959 genes were significantly changed by the parasitism of D. sinensis, including 4769 activated genes and 14,190 suppressed genes. The differentially expressed genes were categorized into 258 KEGG pathways and 647 GO terms. The GO analysis mapped 13 DEGs related to immune system process and 32 DEGs related to immune response, respectively, suggesting a potential alteration of transcriptional expression patterns in complement cascades of P. sanguinolentus. Additionally, 4 representative molting-related genes were down-regulated in parasitized group, indicating D. sinensis infection appeared to suppress the producing of ecdysteroid hormones. In conclusion, the present study improves our understanding on parasite-host interaction mechanisms, which focuses the function of Ecdysone receptor, Toll-like receptor and cytokine receptor of crustacean crabs infestation with rhizocephalan parasites.

Full-Length Transcriptome Analysis Provides New Insights Into the Diversity of Immune-Related Genes in Portunus trituberculatus

Generally, invertebrates were thought to solely rely on their non-specific innate immune system to fight against invading microorganisms. However, increasing studies have implied that the innate immune response of invertebrates displayed diversity and specificity owing to the hyper-variable immune molecules in organisms. In order to get an insight into the diversity of immune-related genes in Portunus trituberculatus, a full-length transcriptome analysis of several immune-related tissues (hemocytes, hepatopancreas and gills) in P. trituberculatus was performed and the diversity of several immune-related genes was analyzed. The full-length transcriptome analysis of P. trituberculatus was conducted using a combination of SMRT long-read sequencing and Illumina short-read sequencing. A total of 17,433 nonredundant full-length transcripts with average length of 2,271 bp and N50 length of 2,841 bp were obtained, among which 13,978 (80.18%) transcripts were annotated. Moreover, numerous transcript variants of various immune-related genes were identified, including pattern recognition receptors, antimicrobial peptides, heat shock proteins (HSPs), antioxidant enzymes and vital molecules in prophenoloxidase (proPO)-activating system. Based on the full-length transcriptome analysis, open reading frames (ORFs) of four C-type lectins (CTLs) were cloned, and tissue distributions showed that the four CTLs were ubiquitously expressed in all the tested tissues, and mainly expressed in hepatopancreas and gills. The transcription of the four CTLs significantly increased in several immune-related tissues (hemocytes, hepatopancreas and gills) of P. trituberculatus challenged with Vibrio alginolyticus and displayed different profiles. Moreover, the four CTLs displayed distinct bacterial binding and antibacterial activities. The recombinant protein PtCTL-1 (rPtCTL-1) and rPtCTL-3 displayed bacterial binding and antibacterial activities against all tested bacteria. rPtCTL-2 only showed bacterial binding and antibacterial activities against V. alginolyticus. No obvious bacterial binding or antibacterial activities for PtCTL-4 was observed against the tested bacteria. This study enriches the transcriptomic information on P. trituberculatus and provides new insights into the innate immune system of crustaceans. Additionally, our study provided candidates of antibiotic agents for the prevention and treatment of bacteriosis.

C-type lectin with a QPN motif from swimming crab Portunus trituberculatus displays broad nonself-recognition ability and functions as an opsonin

In the immune system, C-type lectins, as pattern recognition receptors, have an important function. Carbohydrate-recognition domains (CRDs) endow C-type lectins with the function of recognizing and scavenging non-self factors. In the present study, a new C-type lectin (designated as PtCTL-9 according to the order of discovery) from swimming crab (Portunus trituberculatus) was characterized. QPN (Gln-Pro-Asn) and FHS (Phe-His-Ser) were identified as the key motifs that determine carbohydrate binding. Motif QPN was mutated to QPD (Gln-Pro-Asp) (M1) and EPN (Glu-Pro-Asn) (M2) to study its immune function and for comparative analysis. The results showed that PtCTL-9 displayed broad non-self immunity. PtCTL-9 could also function as an opsonin to promote phagocytosis and the in vitro encapsulation of hemocytes. These results indicated that PtCTL-9 has an extensive nonself-recognition ability, regulates pathogen clearance, and its QPN motif is important in PtCTL-9's immune function.

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.

A New C-Type Lectin Homolog SpCTL6 Exerting Immunoprotective Effect and Regulatory Role in Mud Crab Scylla paramamosain

C-type lectin (CTL), a well-known immune-related molecule, has received more and more attention due to its diverse functions, especially its important role in development and host defense of vertebrate and invertebrate. Since the research on crab CTLs is still lack, we screened a new CTL homolog, named SpCTL6 from mud crab Scylla paramamosain. The full-length cDNA sequence of SpCTL6 was 738 bp with a 486 bp of ORF, and the deduced amino acids were 161 aa. SpCTL6 was predicted to have a 17 aa signal peptide and its mature peptide was 144 aa (MW 16.7 kDa) with pI value of 5.22. It had typical CTL structural characteristics, such as a single C-type lectin-like domain, 4 conserved cysteines, similar tertiary structure to that of vertebrate CTLs and a mutated Ca²⁺ binding motif Gln-Pro-Thr (QPT), clustering into the same branch as the crustacean CTLs. SpCTL6 was highly expressed in the entire zoeal larval stages and widely distributed in adult crab tissues with the highest transcription level in testis. During the molting process of juvenile crabs, the expression level of SpCTL6 was remarkably increased after molting. SpCTL6 could be significantly upregulated in two larval stages (Z1 and megalopa) and adult crab testis under immune challenges. Recombinant SpCTL6 (rSpCTL6) was successfully obtained from eukaryotic expression system. rSpCTL6 exhibited binding activity with PAMPs (LPS, lipoteichoic acid, peptidoglycan, and glucan) and had a broad spectrum bacterial agglutination activity in a Ca²⁺-dependent manner. In addition, rSpCTL6 could enhance the encapsulation activity of hemocytes and has no cytotoxic effect on hemocytes. Although rSpCTL6 had no bactericidal activity on Vibrio alginolyticus, rSpCTL6 treatment could significantly reduce the bacterial endotoxin level in vitro and greatly improved the survival of S. paramamosain under V. alginolyticus infection in vivo. The immunoprotective effect of rSpCTL6 might be due to the regulatory role of rSpCTL6 in immune-related genes and immunological parameters. Our study provides new information for understanding the immune defense of mud crabs and would facilitate the development of effective strategies for mud crab aquaculture disease control.

Antimicrobial properties and immune-related gene expression of a C-type lectin isolated from Pinctada fucata martensii

C-type lectins are carbohydrate-binding proteins that play important roles in the innate immune response to pathogen infections. Here, multi-step high performance liquid chromatography (HPLC), combined with mass spectrometry (MS), was used to isolate and identify proteins with antibacterial activity from the serum of Pinctada fucata martensii. Using this method, we obtained a novel isoform of C-type lectin (PmCTL-1). PmCTL-1 strongly inhibited gram-positive bacteria. The complete cDNA sequence of PmCTL-1 was 636 bp in length, and encoded a protein 149 amino acids long, containing a typical carbohydrate recognition domain (CRD). A phylogenetic analysis based on a multiple sequence alignment indicated that PmCTL-1 was highly similar to C-type lectins from other mollusks. Fluorescent quantitative real-time PCR analysis showed that PmCTL-1 mRNA was strongly upregulated in the mantle of healthy P.f. martensii, but was expressed only at low levels in the gill, gonad, hepatopancreas, adductor muscle, and hemocytes. PmCTL-1 expression levels in the mantle and hemocytes increased significantly in response to bacterial stimulation. This study provides a valuable framework for further explorations of innate immunity and the immune response in mollusks.

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.

Two C-type lectins (ReCTL-1, ReCTL-2) from Rimicaris exoculata display broad nonself recognition spectrum with novel carbohydrate binding specificity

C-type lectins are Ca²⁺-dependent carbohydrate-binding proteins containing one or more carbohydrate-recognition domains (CRDs). C-type lectins play crucial roles in innate immunity, including nonself-recognition and pathogen elimination. In the present study, two C-type lectins (designated ReCTL-1 and ReCTL-2) were identified from the shrimp Rimicaris exoculata which dwells in deep-sea hydrothermal vents. The open reading frames of ReCTL-1 and ReCTL-2 encoded polypeptides of 171 and 166 amino acids respectively, which were both composed of a signal peptide and a single CRD. The key motifs determining the carbohydrate binding specificity of ReCTL-1 and ReCTL-2 were respectively Glu-Pro-Ala (EPA) and Gln-Pro-Asn (QPN), which were firstly discovered in R. exoculata. ReCTL-1 and ReCTL-2 displayed similar pathogen-associated molecular pattern (PAMP) binding features and they bound three PAMPs-β-glucan, lipopolysaccharide and peptidoglycan-with relatively high affinity. In addition, both could efficiently recognize and bind Gram-positive bacteria, Gram-negative bacteria and fungi. However, ReCTL-1 and ReCTL-2 exhibited different microbial agglutination activities: ReCTL-1 agglutinated Staphylococcus aureus and Saccharomyces cerevisiae, while ReCTL-2 agglutinated Micrococcus luteus, Vibrio parahaemolyticus and V. fluvialis. Both ReCTL-1 and ReCTL-2 inhibited the growth of V. fluvialis. All these results illustrated that ReCTL-1 and ReCTL-2 could function as important pattern-recognition receptors with broad nonself-recognition spectra and be involved in immune defense against invaders, but their specificities are not the same. In addition, the two ReCTLs possessed different carbohydrate binding specificities from each other and from the classical pattern: ReCTL-1 with an EPA motif bound d-galactose and l-mannose, while ReCTL-2 with a QPN motif bound d-fucose and N-acetylglucosamine.

A novel C-type lectin from spotted knifejaw, Oplegnathus punctatus possesses antibacterial and anti-inflammatory activity

C-type lectin is a type of carbohydrate-binding protein and plays significant roles in innate immune response against pathogen infection. To date, thousands of C-type lectin had been identified in teleost. In the present study, we isolated a novel isoform of C-type lectin (OppCTL) from spotted knifejaw (Oplegnathus punctatus). The OppCTL encoded a typical Ca²⁺-dependent carbohydrate-binding protein, and was mainly expressed in liver in a tissue specific fashion. The expression of OppCTL was significantly up-regulated following Vibrio anguillarum infection in vivo, suggesting involvement in immune response. Hemagglutination analysis showed that the recombinant OppCTL (rOppCTL) could agglutinate erythrocyte from Mus musculus, Oplegnathus punctatus, Sebastes schlegelii and Paralichthys olivaceus. The rOppCTL could bind and agglutinate all tested bacteria. The rOppCTL possessed capacities of calcium-dependent agglutination to all tested bacteria. Sugar binding assay revealed that rOppCTL could also bind to the glycoconjugates of the bacterial surface, including lipopolysaccharide and peptidoglycan. Interestingly, Dual-luciferase analysis revealed that OppCTL could inhibit the activity of NF-κB in HEK-293T cells after OppCTL overexpression. Taken together, these results indicate that OppCTL has immune activity capable of defending invading pathogens and possesses potential immunoregulatory activity, enriching our understanding of the function of C-type lectin.

Novel Ca2+-independent C-type lectin involved in immune defense of the razor clam Sinonovacula constricta

C-type lectins (CTLs) are important pattern recognition molecules that participate in bacterial binding and agglutination by specific recognition of carbohydrates from pathogens. In this study, a full-length cDNA of CTL was cloned from Sinonovacula constricta (designated ScCTL-2). ScCTL-2 has a length of 981 bp, a 5'-untranslated region (UTR) of 47 bp, a short 3'-UTR of 37 bp, and an open reading frame (ORF) of 894 bp, which encodes a polypeptide of 298 amino acid residues. The deduced amino acid of ScCTL-2 possesses a conserved carbohydrate-recognition domain (CRD) similar to that of C³¹-E¹⁷¹. Spatial distribution analysis demonstrated that ScCTL-2 was constitutively expressed in all tested tissues, with dominant expression in foot and siphon and weak expression in hepatopancreas. The mRNA expression level of ScCTL-2 in gills and hepatopancreas was significantly upregulated at 6 and 12 h after challenge with the pathogen Vibrio parahaemolyticus. The recombinant ScCTL-2 showed specific binding and agglutinate capacities to all examined Gram-negative bacterial species, namely, Escherichia coli, Vibro anguillarum, and V. parahaemolyticus in a Ca²⁺-independent manner. However, these binding activities were not detected in Gram-positive Micrococcus luteus. Our results indicated that ScCTL-2 could be a novel pattern recognition receptor that can specifically recognize Gram-negative microorganisms in the innate immunity of S. constricta.

First characterization of two C-type lectins of the tubeworm Alaysia sp. from a deep-sea hydrothermal vent

C-type lectins (CTLs) play an important role in innate immune defense. In this study, we identified and characterized two CTLs (Lec1 and Lec2) from the tubeworm Alaysia sp. collected from a hydrothermal vent in Pacmanus. Lec1 and Lec2 possess the typical CTL domain but share low sequence identities (10.8%-20.4%) with known CTLs. Recombinant (r) of Lec1 and Lec2 bound to various PAMPs and a wide arrange of bacteria from neritic and deep-sea environments in a Ca²⁺-independent manner, but only rLec1 caused agglutination of the bound bacteria. The activities of rLec1 and rLec2 were most stable and highest at 4 °C, the ambient temperature of the hydrothermal vent, and decreased at higher temperatures. Both lectins inhibited bacterial growth in a highly selective manner and agglutinated the erythrocytes of fish, rabbit, and chicken in a Ca²⁺-dependent manner. These results provided the first insights into the functional properties of CTLs in deep-sea Alaysia sp.

Characterisation and functional comparison of single-CRD and multidomain containing galectins CgGal-2 and CgGal-3 from oyster Crassostrea gigas

Galectins are β-galactoside binding lectins that play crucial roles in innate immunity in vertebrates and invertebrates through their conserved carbohydrate-recognition domains (CRDs). In the present study, single- and four-CRD-containing galectins were identified in oyster Crassostrea gigas (designated CgGal-2 and CgGal-3). The open reading frames (ORFs) of CgGal-2 and CgGal-3 encode polypeptides of 200 and 555 amino acids, respectively. All CRDs of CgGal-3 include two consensus motifs essential for ligand-binding, and a novel motif is present in CgGal-2. Pathogen-associated molecular pattern (PAMP) profiles were determined for recombinant rCgGal-2 and rCgGal-3, and rCgGal-2 displayed low binding affinity for PAMPs, while rCgGal-3 bound various PAMPs including glucan, lipopolysaccharide (LPS), and peptidoglycan (PGN) with relatively high affinity. Furthermore, rCgGal-2 and rCgGal-3 exhibited different microbe binding profiles; rCgGal-2 bound to Gram-negative bacteria (Escherichia coli and Vibrio vulnificus) and fungi (Saccharomyces cerevisiae and Pichia pastoris), while rCgGal-3 bound to these microbes but also to Gram-positive bacteria (Micrococcus luteus). In addition, rCgGal-3 possessed microbial agglutinating activity and coagulation activity against fungi and erythrocytes, respectively, but rCgGal-2 lacked any agglutinating activity. Carbohydrate binding specificity analysis showed that rCgGal-3 specifically bound D-galactose. Furthermore, rCgGal-2 and rCgGal-3 functioned as opsonin participating in the clearance against invaders in C. gigas. Thus, CgGal-2 with one CRD and CgGal-3 with four CRDs are new members of the galectin family involved in immune responses against bacterial infection. Differences in the organisation and amino acid sequences of CRDs may affect their specificity and affinity for nonself substances.

Molecular cloning of a C-type lectin from Portunus trituberculatus, which might be involved in the innate immune response

C-type lectin plays an important role in the innate immune response of crustaceans including Portunus trituberculatus which is an important marine species. In the present study, we cloned the full length of a C-type lectin (designated as PtCTL4) from P. trituberculatus via 3'RACE. The full length of the nucleic acid sequence has a length of 654 bp including an open reading frame (ORF) of 480 bp. PtCTL4 possesses conserved CTL features, while containing a CRD domain with Ca²⁺ binding site 2 and six conserved cysteine residues. Quantitative RT-PCR analysis showed that PtCTL4 expression level was highest in the hepatopancreas, while it was relatively low in other tissues such as hemocytes, eyestalk, muscle, and gonad. The expression level of PtCTL4 reached a maximum at 3 h after challenge with Vibrio alginolyticus, then decreased to the lowest level at 12 h, and returned to normal level at 48 h. Hemagglutination analysis showed that the recombinant PtCTL4 (rPtCTL4) can agglutinate rabbit erythrocyte. The rPtCTL4 can agglutinate Gram-positive bacteria (Bacillus aquimaris, Micrococcus lysodeik, and Staphylococcus aureus) and Gram-negative bacteria (Aeromonas hydrophila, V. alginolyticus, and Chryseobacterium indologenes) in the presence of Ca²⁺. This study indicated that PtCTL4 acts as a pattern recognition receptor in the innate immune response of P. trituberculatus.