l-rhamnose-binding lectins (RBLs) in turbot (Scophthalmus maximus L.): Characterization and expression profiling in mucosal tissues

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.

Effects of chilling and cryoprotectants on glycans in shrimp embryos

Glycans are carbohydrates present in every organism that bind to specific molecules such as lectins, a diverse group of proteins. Glycans are vital to cell proliferation and protein trafficking. In addition, embryogenesis is a critical phase in the development of marine organisms. This study investigated the effects of chilling and cryoprotective agents (CPAs) on glycans in the embryos of Stenopus hispidus. The glycan profiles of embryos of S. hispidus at the heartbeat stage were analyzed using lectin arrays. The results of analyses revealed that mannose was the most abundant glycan in the S. hispidus embryos; mannose is crucial to cell proliferation, providing the energy required for embryonic growth. Additionally, the results reveled that chilling altered the content of several glycans, including fucose and Gla-GlcNAc. Chilling may promote monosaccharide accumulation, facilitating osmotic regulation of cells and signal molecules to aid S. hispidus embryos in adapting to cold conditions. Changes were also observed in the lectins NPA, orysata, PALa, ASA, discoidin II, discoidin I, UDA, PA-IIL, and PHA-P after the samples were treated with different CPAs. DMSO may minimize cell damage during exposure to chilling by preserving cell structures, membrane properties, and functions. The present study is the first to investigate the profiles and functions of glycans in shrimp embryos subjected to low-temperature injuries. This study enhances the understanding of cell reproduction during embryogenesis and provides valuable information for the study of glycans in embryos.

New l-Rhamnose-Binding Lectin from the Bivalve Glycymeris yessoensis: Purification, Partial Structural Characterization and Antibacterial Activity

In this study, a new l-rhamnose-binding lectin (GYL-R) from the hemolymph of bivalve Glycymeris yessoensis was purified using affinity and ion-exchange chromatography and functionally characterized. Lectin antimicrobial activity was examined in different ways. The lectin was inhibited by saccharides possessing the same configuration of hydroxyl groups at C-2 and C-4, such as l-rhamnose, d-galactose, lactose, l-arabinose and raffinose. Using the glycan microarray approach, natural carbohydrate ligands were established for GYL-R as l-Rha and glycans containing the α-Gal residue in the terminal position. The GYL-R molecular mass determined by MALDI-TOF mass spectrometry was 30,415 Da. The hemagglutination activity of the lectin was not affected by metal ions. The lectin was stable up to 75 °C and between pH 4.0 and 12.0. The amino acid sequence of the five GYL-R segments was obtained with nano-ESI MS/MS and contained both YGR and DPC-peptide motifs which are conserved in most of the l-rhamnose-binding lectin carbohydrate recognition domains. Circular dichroism confirmed that GYL is a α/β-protein with a predominance of the random coil. Furthermore, GYL-R was able to bind and suppress the growth of the Gram-negative bacteria E. coli by recognizing lipopolysaccharides. Together, these results suggest that GYL-R is a new member of the RBL family which participates in the self-defense mechanism against bacteria and pathogens with a distinct carbohydrate-binding specificity.

New Data on the Rhamnose-Binding Lectin from the Colonial Ascidian Botryllus schlosseri: Subcellular Distribution, Secretion Mode and Effects on the Cyclical Generation Change

Botryllus schlosseri in a cosmopolitan ascidian, considered a reliable model organism for studies on the evolution of the immune system. B. schlosseri rhamnose-binding lectin (BsRBL) is synthesised by circulating phagocytes and behaves as an opsonin by interacting with foreign cells or particles and acting as a molecular bridge between them and the phagocyte surface. Although described in previous works, many aspects and roles of this lectin in Botryllus biology remain unknown. Here, we studied the subcellular distribution of BsRBL during immune responses using light and electron microscopy. In addition, following the hints from extant data, suggesting a possible role of BsRBL in the process of cyclical generation change or takeover, we investigated the effects of interfering with this protein, by injecting a specific antibody in the colonial circulation, starting one day before the generation change. Results confirm the requirement of the lectin for a correct generation change and open new queries on the roles of this lectin in Botryllus biology.

A novel l-rhamnose-binding lectin participates in defending against bacterial infection in zebrafish

l-rhamnose-binding lectin (RBL), which is a class of animal lectins independent of Ca²⁺, can specifically bind l-rhamnose or d-galactose. Although several lectins in zebrafish have been reported, their functional mechanisms have not been fully uncovered. In this study, we discovered a novel l-rhamnose binding lectin (DrRBL) and studied its innate immune function. The DrRBL protein contains only one carbohydrate-recognition domain (CRD), which includes two strictly conserved motifs, "YGR" and "DPC". DrRBL was detected in all tested tissues and was present at high levels in the spleen, hepatopancreas and skin. After Aeromonas hydrophila challenge, the DrRBL mRNA level was significantly upregulated. Additionally, DrRBL was secreted into the extracellular matrix. Recombinant DrRBL (rDrRBL) could significantly inhibit the growth of gram-positive/negative bacteria, bind to several bacteria and cause obvious agglutination. The rDrRBL protein could combine with polysaccharides, such as PGN and LPS, rather than LTA. A more detailed study showed that rDrRBL could combine with monosaccharides, such as mannose, rhamnose and glucose, which are important components of PGN and LPS. However, rDrRBL could not bind to ribitol, which is an important component of LTA. The DrRBL deletion mutants, DrRBL^Δ144-150 and DrRBL^Δ198-200, were also constructed. DrRBL^Δ144-150 ("ANYGRTD" deficient) showed weak bacterial inhibiting ability. However, DrRBL^Δ198-200 ("DPC" deficient) showed weak agglutination ability. These results suggest that the "DPC" domain is important for agglutination. The conserved domain "ANYGRTD" is essential for inhibiting bacterial growth.

Systematic identification and characterization of lncRNAs and lncRNA-miRNA-mRNA networks in the liver of turbot (Scophthalmus maximus L.) induced with Vibrio anguillarum

Long noncoding RNAs (lncRNAs), can regulate mRNA by targeting miRNA in a competing endogenous RNA network, have become a hot topic in the research of fish immune mechanism recent years. While in turbot (Scophthalmus maximus L.), an economically important marine fish, there are limited researches about the role of lncRNAs in its immune response to bacterial infection. In this study, a total of 184 differentially expressed lncRNAs (DElncRNAs) were systematically identified and characterized using whole-transcriptome sequencing of the liver of turbot challenged with Vibrioanguillarum at 0 h (control) and three different time points post infection (2 h, 12 h and 24 h, respectively). Subsequently, GO and KEGG signaling pathways of differentially expressed lncRNAs were analyzed to predict their function. We found that lncRNAs in our results were significantly enriched in several immune-related signaling pathways, including the NOD-like receptor signaling pathway, Toll-like receptor signaling pathway, Cytokine-cytokine receptor, MAPK signaling pathway, phagosome, PPAR signaling pathway and the regulation of autophagy. In addition, a total of 492 DE lncRNA - DE miRNA -DE mRNA networks were identified at three different time points post infection, which were consisted of 102 networks at 2 h, 122 networks at 12 h and 81 networks at 24 h post infection, respectively. Noticeably, 92 of these regulated networks were immune-related. These observations suggested that lncRNAs can regulate the expression of immune-related genes in the response to bacterial infection in turbot. Moreover, our findings would provide a new insight into the immune response of turbot to pathogen infection and lay a foundation for future study.

Comprehensive identification and expression profiling of immune-related lncRNAs and their target genes in the intestine of turbot (Scophthalmus maximus L.) in response to Vibrio anguillarum infection

Long non-coding RNA (lncRNA) play vital regulatory roles in various biological processes. Intestine is one of the most sensitive organs to environmental and homeostatic disruptions for fish. However, systematic profiles of lncRNAs in the intestine of teleost in responses to pathogen infections is still limited. Turbot (Scophthalmus maximus L.), an important commercial fish species in China, has been suffering with Vibrio anguillarum infection, resulted in dramatic economic loss. Hereinto, the intestinal tissues of turbot were sampled at 0 h, 2 h, 12 h, and 48 h following V. anguillarum infection. The histopathological analysis revealed that the pathological trauma was mainly present in intestinal tunica mucosal epithelium. After high-throughput sequencing and bioinformatic analysis, a total of 9722 lncRNAs and 21,194 mRNAs were obtained, and the average length and exon number of lncRNAs were both less than those of mRNAs. Among which, a set of 158 lncRNAs and 226 mRNAs were differentially expressed (DE-lncRNAs and DEGs) in turbot intestine at three time points, related to many immune-related genes such as complement, interleukin, chemokine, lysosome, and macrophage, indicating their potential critical roles in immune responses. In addition, 2803 and 1803 GO terms were enriched for DEGs and co-expressed target genes of DE-lncRNAs, respectively. Moreover, 127 and 50 KEGG pathways including cell adhesion molecules (CAMs), phagosome, JAK-STAT signaling pathway, cytokine-cytokine receptor interaction, and intestinal immune network for IgA production, were enriched for DEGs and co-expressed target genes of DE-lncRNAs, respectively. Finally, qRT-PCR was conducted to confirm the reliability of sequencing data. The present study will set the foundation for the future exploration of lncRNA functions in teleost in response to bacterial infection.

An Update of Lectins from Marine Organisms: Characterization, Extraction Methodology, and Potential Biofunctional Applications

Lectins are a unique group of nonimmune carbohydrate-binding proteins or glycoproteins that exhibit specific and reversible carbohydrate-binding activity in a non-catalytic manner. Lectins have diverse sources and are classified according to their origins, such as plant lectins, animal lectins, and fish lectins. Marine organisms including fish, crustaceans, and mollusks produce a myriad of lectins, including rhamnose binding lectins (RBL), fucose-binding lectins (FTL), mannose-binding lectin, galectins, galactose binding lectins, and C-type lectins. The widely used method of extracting lectins from marine samples is a simple two-step process employing a polar salt solution and purification by column chromatography. Lectins exert several immunomodulatory functions, including pathogen recognition, inflammatory reactions, participating in various hemocyte functions (e.g., agglutination), phagocytic reactions, among others. Lectins can also control cell proliferation, protein folding, RNA splicing, and trafficking of molecules. Due to their reported biological and pharmaceutical activities, lectins have attracted the attention of scientists and industries (i.e., food, biomedical, and pharmaceutical industries). Therefore, this review aims to update current information on lectins from marine organisms, their characterization, extraction, and biofunctionalities.

An l-rhamnose-binding lectin from Nile tilapia (Oreochromis niloticus) possesses agglutination activity and regulates inflammation, phagocytosis and respiratory burst of monocytes/macrophages

Rhamnose-binding lectins (RBLs), a Ca²⁺-independent lectin family, are widely present in vertebrates and invertebrates, which involve in the innate immune response. However, the functional characterization and related regulation mechanisms of RBLs remain unclear in teleost fish. In this study, an l-rhamnose-binding lectin-like (OnRBL-L) was identified and functionally characterized from Nile tilapia (Oreochromis niloticus). The open reading frame of OnRBL-L is 678 bp encoding 225 aa. The sequence of OnRBL-L has relatively conservative characteristic peptide motifs, including YGR, DPC, and KYL-motif. Expression analysis showed that OnRBL-L was abundantly distributed in intestine tissue, and widely existed in all detected tissues. Meanwhile, the expression of OnRBL-L increased significantly in vivo (liver, spleen, head kidney, intestine, gills and peripheral blood) and in vitro (monocytes/macrophages) following challenges with two important tilapia pathogenic bacteria Streptococcus agalactiae and Aeromonas hydrophila. In addition, the recombinant OnRBL-L was found to bind and agglutinate S. agalactiae and A. hydrophila. Furthermore, OnRBL-L could participate in non-specific cellular immune defense, including reducing the expression of pro-inflammatory factors (IL-6、IL-8 and TNF-α), and enhancement of the phagocytosis and respiratory burst of MO/MФ. Overall, our results provide new insights into the understanding of RBL as an important pattern recognition molecule and regulator in non-specific cell immunity in an early vertebrate.

Comparative analysis of the miRNA-mRNA regulation networks in turbot (Scophthalmus maximus L.) following Vibrio anguillarum infection

MicroRNAs could not only regulate posttranscriptional silencing of target genes in eukaryotic organisms, but also have positive effect on their target genes as well. These microRNAs have been reported to be involved in mucosal immune responses to pathogen infection in teleost. Therefore, we constructed the immune-related miRNA-mRNA networks in turbot intestine following Vibrio anguillarum infection. In our results, 1550 differentially expressed (DE) genes and 167 DE miRNAs were identified. 113 DE miRNAs targeting 89 DE mRNAs related to immune response were used to construct miRNA-mRNA interaction networks. Functional analysis showed that target genes were associated with synthesis and degradation of ketone bodies, mucin type O-Glycan biosynthesis, homologous recombination, biotin metabolism, and intestinal immune network for IgA production that were equivalent to the function of IgT and IgM in fish intestine. Finally, 10 DE miRNAs and 7 DE mRNAs were selected for validating the accuracy of high-throughput sequencing results by qRT-PCR. The results of this study will provide valuable information for the elucidation of the regulation mechanisms of miRNA-mRNA interactions involved in disease resistance in teleost mucosal immune system.

Antibacterial functions of a novel fish-egg lectin from spotted knifejaw (Oplegnathus punctatus) during host defense immune responses

Fish-egg lectins (FELs) have been identified in several teleost species and have been proved to play important roles in innate immune system against pathogen infection. In this study a novel fish-egg lectin (OppFEL) was identified from spotted knifejaw (Oplegnathus punctatus), and the expression patterns against bacterial infection was characterized. The amino acid sequence is highly homologous to other teleost FELs, containing five repeats of the conserved TECPR domain. Expression of OppFEL was widely observed in examined tissues, with the most abundant transcripts observed in gill, showing a pattern of tissue specific expression. The OppFEL expression was significantly up-regulated following a Gram-negative bacterium (Vibrio anguillarum) challenge in vivo, suggesting participation in host antibacterial immune responses. Recombinant OppFEL protein (rOppFEL) possessed calcium dependent binding capacities and agglutination to four Gram-negative bacterium and two Gram-positive bacterium. Sugar binding assay revealed that rOppFEL specifically bound to insoluble lipopolysaccharide and peptidoglycan. In addition, rOppFEL was also proved to have hemagglutinating activity against erythrocytes from Mus musculus, O. punctatus, Sebastes schlegelii and Paralichthys olivaceus. Dual-luciferase analysis showed that overexpression of OppFEL could suppress the activity of NF-κB in a dose dependent manner. Taken together, these results suggest that OppFEL is a unique fish-egg lectin that possesses apparent immunomodulating property and is involved in host defense against pathogens invasion.

Characterization and initial functional analysis of cathepsin K in turbot (Scophthalmus maximus L.)

Cathepsins are the best-known group of proteases in lysosomes, playing a significant role in immune responses. Cathepsin K (CTSK) is abundantly and selectively expressed in osteoclasts, dendritic cells and monocyte-derived macrophages, where it is involved in ECM degradation and bone remodeling. A growing body of evidences have indicated the vital roles of cathepsin K in innate immune responses. Here, one CTSK gene was captured in turbot (SmCTSK) with a 993 bp open reading frame (ORF). The genomic structure analysis showed that SmCTSK had 7 exons similar to other vertebrate species. The syntenic analysis revealed that CTSK had the same neighboring genes across all the selected species, which suggested the synteny encompassing CTSK region was conserved during vertebrate evolution. Subsequently, SmCTSK was widely expressed in all the examined tissues, with the highest expression level in spleen and the lowest expression level in liver. In addition, SmCTSK was significantly down-regulated in intestine following Gram-negative bacteria Vibrio anguillarum immersion challenge, but up-regulated in three tissues (gill, skin and intestine) following Gram-positive bacteria Streptococcus iniae immersion challenge. Finally, the rSmCTSK showed strong binding ability to all the examined microbial ligands. Taken together, our results suggested SmCTSK played vital roles in fish innate immune responses against infection. However, the knowledge of SmCTSK is still limited in teleost species, further studies should be carried out to better characterize its comprehensive roles in teleost mucosal immunity.

Identification and initial functional characterization of lysosomal integral membrane protein type 2 (LIMP-2) in turbot (Scophthalmus maximus L.)

The immune system protects organism from external pathogens, this progress starts with the pathogen recognition by pattern recognition receptors (PRRs). As a group of PRRs, the class B scavenger receptors showed important roles in phagocytosis. Among three class B scavenger receptors, lysosomal integral membrane protein type 2 (LIMP-2) was reported to present in the limiting membranes of lysosomes and late endosomes, but its immune roles in teleost species are still limited in handful species. Here, we characterized LIMP-2 gene in turbot, and its expression patterns in mucosal barriers following different bacterial infection, as well as ligand binding activities to different microbial ligands and agglutination assay with different bacteria. In our results, one SmLIMP2 gene was identified with a 1,593 bp open reading frame (ORF). The multiple species comparison and phylogenetic analysis showed the closest relationship to Paralichthys olivaceus, the genomic structure analysis and syntenic analysis revealed the conservation of LIMP-2 during evolution. In tissue distribution analysis, SmLIMP-2 was expressed in all the examined turbot tissues, with the highest expression level in brain, and the lowest expression level in liver. In addition, SmLIMP-2 was significantly up-regulated in all the mucosal tissues (skin, gill and intestine) following Gram-negative bacteria Vibrio anguillarum infection, and was only up-regulated in gill following Gram-positive bacteria Streptococcus iniae challenge. Finally, the rSmLIMP-2 showed strong binding ability to all the examined microbial ligands, and strong agglutination with Escherichia coli, Staphylococcus aureus and V. anguillarum. Taken together, our results suggested SmLIMP-2 played important roles in fish immune response to bacterial infection. However, further functional studies should be carried out to better characterize its detailed roles in teleost immunity.

Characterization, expression profiling and functional characterization of cathepsin Z (CTSZ) in turbot (Scophthalmus maximus L.)

Cathepsin Z (CTSZ) is a lysosomal cysteine protease of the papain superfamily. It participates in the host immune defense via phagocytosis, signal transduction, cell-cell communication, proliferation, and migration of immune cells such as monocytes, macrophages, and dendritic cells. In this study, we reported the identification of SmCTSZ, a CTSZ homolog from turbot (Scophthalmus maximus L.). SmCTSZ was 317 residues in length and contains a Pept-C1 domain. In multiple species comparison, SmCTSZ shared 65-93% overall sequence identities with the CTSZ counterparts from human, rat, and several fish species. In the phylogenetic analysis, SmCTSZ showed the closest relationship to Cynoglossus semilaevis. The syntenic analysis revealed the similar neighboring genes of CTSZ across all the selected species, which suggested the synteny encompassing CTSZ region during vertebrate evolution. Subsequently, SmCTSZ was constitutively expressed in various tissues, with the lowest and highest levels in brain and intestine respectively. In addition, SmCTSZ was significantly up-regulated in intestine following both Gram-negative bacteria Vibrio anguillarum, and Gram-positive bacteria Streptococcus iniae immersion challenge. Finally, the rSmCTSZ showed strong binding ability to all the examined microbial ligands, and the agglutination effect to different bacteria. Taken together, these results indicated SmCTSZ could play important roles in mucosal immune response in the event of bacterial infection in teleost. However, the knowledge of CTSZ are still limited in teleost species, further studies should be carried out to better characterize its detailed roles in teleost mucosal immunity.

Expression profiling and functional characterization of CD36 in turbot (Scophthalmus maximus L.)

CD36 is a scavenger receptor, a type of membrane-bound receptors that characterized by recognizing a variety of ligands including endogenous proteins and pathogens. Here, we characterized CD36 gene in turbot, and its expression patterns in mucosal barriers following different bacterial infection, as well as microbial ligand binding ability and bacteriostatic activities. In current study, one SmCD36 gene was captured with a 1407 bp open reading frame (ORF). In multiple species comparison, SmCD36 showed the highest similarity and identity to Cynoglossus semilaevis. In the phylogenetic analysis, SmCD36 showed the closest relationship to C. semilaevis, followed by Takifugu rubripes. The genomic structure analysis showed that CD36 had 12 exons with almost the same length in vertebrate species, indicating the conservation of CD36 during evolution. The syntenic analysis revealed that CD36 located between GNAI1 and SEMA3C genes across all the selected species, which suggested the synteny encompassing CD36 region during vertebrate evolution. Subsequently, SmCD36 was expressed in all the examined turbot tissues, with the highest expression level in intestine. In addition, SmCD36 was significantly up-regulated in intestine following both Gram-negative bacteria Vibrio anguillarum, and Gram-positive bacteria Streptococcus iniae immersion challenge. Finally, the rSmCD36 showed strong binding ability to all the examined microbial ligands and significant inhibition effect on Staphylococcus aureusrequires. Taken together, our results suggested SmCD36 involved in fish innate immune responses to bacterial infection. However, the knowledge of CD36 are still limited in teleost species, further studies should be carried out to better characterize its detailed roles in teleost mucosal immunity.