A C-type lectin from the tunicate, Styela plicata, that modulates cellular activity

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.

Molecular cloning and expression analysis of C-type lectin (RpCTL) in Manila clam Ruditapes philippinarum after lipopolysaccharide challenge

The Manila clam, Ruditapes philippinarum, is one of the most commercially important marine bivalves. C-type lectins (CTLs) are pattern recognition receptors (PRRs) that play important roles in the identification and elimination of pathogens by the innate immune system. In this study, a new CTL (RpCTL) was identified in the Manila clam, R. philippinarum. The full-length RpCTL cDNA is 802 bp, with an open reading frame of 591 bp, encoding 196 amino acids, including an N-terminal signal peptide and a carbohydrate recognition domain (CRD). RpCTL contains conserved CRD disulfide bonds involving four cysteine residues (Cys³⁰-Cys¹⁰⁴, Cys¹²⁴, and Cys¹³²), and the EPN (Glu⁹⁴-Pro⁹⁵-Asn⁹⁶) and WND (Trp¹¹⁹-Asn¹²⁰-Asp¹²¹) motifs. Quantitative reverse transcription (RT)-PCR detected RpCTL transcripts mainly in the gill, siphon, and hepatopancreas in three shell-color strains (zebra, white, and white-zebra strains) and two unselected populations of R. philippinarum, and the gene was highly expressed in the hepatopancreas after lipopolysaccharide treatment. Antimicrobial activity assays of recombinant RpCTL against both Gram-positive and Gram-negative bacteria showed that RpCTL inhibits microorganismal growth. In a survival test, RpCTL inhibited and killed Vibrio anguillarum in R. philippinarum. These results suggest that RpCTL participates in the pathogen identification process of R. philippinarum as a PRR and in its immune defense system.

Molecular cloning and expression of a C-type lectin gene from Venerupis philippinarum

C-type lectins have been demonstrated to play important roles in invertebrate innate immunity by mediating the recognition of pathogens and clearing the micro-invaders. In the present study, a C-type lectin gene (denoted as VpCTL) was identified from Venerupis philippinarum by expressed sequence tag and rapid amplification of cDNA ends approaches. The full-length cDNA of VpCTL consists of 904 nucleotides with an open-reading frame of 456 bp encoding a peptide of 151 amino acids. The deduced amino acid sequence of VpCTL shared high similarity with C-type lectins from other species. The C-type lectin domain and the characteristic EPN and WND motifs were found in VpCTL. The VpCTL mRNA was dominantly expressed in the haemocytes of the V. philippinarum. After Listonella anguillarum challenge, the temporal expression of VpCTL mRNA in haemocytes was increased by 97- and 84-fold at 48 and 96 h, respectively. With high expression level in haemocytes and hepatopancreas, and the up-regulated expression in haemocytes indicted that VpCTL was perhaps involved in the immune responses to L. anguillarum challenge.

A C-type lectin (AiCTL-3) from bay scallop Argopecten irradians with mannose/galactose binding ability to bind various bacteria

C-type lectins are a family of Ca(2+)-dependent carbohydrate-binding proteins playing crucial roles in innate immunity of vertebrates and invertebrates. In the present study, the cDNA of a C-type lectin with one carbohydrate-recognition domain (CRD) of 127 amino acids was cloned from bay scallop Argopecten irradians (designated AiCTL-3) by rapid amplification of cDNA end (RACE) techniques based on expressed sequence tag (EST) analysis. The mRNA transcripts of AiCTL-3 could be detected in all the tested tissues including hepatopancreas, gonad, adductor muscle, heart, hemocytes, mantle and gill, with the highest expression level in hepatopancreas. After the challenges with Vibrio anguillarum and Micrococcus luteus, the mRNA expression level of AiCTL-3 was obviously up-regulated and reached the maximum level at 9h (11.87fold, P<0.01, and 20.02-fold, P<0.05, respectively). The recombinant AiCTL-3 (designated as rAiCTL-3) could bind LPS, PGN, and glucan in vitro, but could not bind mannan. And it also bound Gram-positive bacteria Staphylococcus aureus as well as Gram-negative bacteria Escherichia coli and V. anguillarum. With a Ca(2+) binding site 2 EPN (Glu-Pro-Asn) motif, rAiCTL-3 could bind both mannose and galactose which was quite different from those in vertebrate. Meanwhile, it could significantly enhance the phagocytosis of scallop hemocytes in vitro. The results clearly suggested that AiCTL-3 could serve not only as a PRR participated in the immune response against various PAMPs and bacteria in non-self recognition via mannose/galactose binding specificity but an opsonin playing an important part in clearance of invaders.

Lectins of marine hydrobionts

Data from the literature and results of our research on lectins isolated from some kinds of marine hydrobionts such as clams, ascidians, sea worms, sponges, and algae are presented in this review. Results of comparative analysis of the basic physicochemical properties and biological activity of lectins isolated from various sources are discussed.

Nuclear translocation of immulectin-3 stimulates hemocyte proliferation

Immulectin-3 (IML-3) is a C-type lectin from the tobacco hornworm Manduca sexta that contains a motif (NWGV) similar to the BH1 motif (NWGR) of the mammalian galectin-3. IML-3 is synthesized in fat body and secreted into hemolymph, but can be translocated into hemocytes. In this study, we showed that IML-3 was predominantly localized to the nucleus of hemocytes and some metaphase, anaphase and telophase hemocytes from M. sexta larvae injected with bacterial lipopolysaccharide (LPS). IML-3 was detected in the membrane and soluble extracts of hemocytes, suggesting that it may be translocated into hemocytes via receptor-mediated endocytosis. To investigate the role of IML-3 translocation to the nucleus, we expressed recombinant wild-type IML-3 and a deletion mutant DeltaIML-3 that has the NWGV motif deleted in Drosophila S2 cells. We found that recombinant wild-type IML-3, but not DeltaIML-3, was localized to the nucleus of some S2 cells and also detected in the nuclear extract. Expression of recombinant wild-type IML-3, but not DeltaIML-3 or GFP, increased the number of proliferating S2 cells. Our results suggest that nuclear translocation of IML-3 may stimulate hemocyte proliferation.

Lectin from the Manila Clam Ruditapes philippinarum Is Induced upon Infection with the Protozoan Parasite Perkinsus olseni

Glycan-binding proteins (lectins) are widely expressed in many invertebrates, although the biosynthesis and functions of the lectins are not well understood. Here we report that Manila clam (Ruditapes philippinarum) synthesizes a lectin termed Manila clam lectin (MCL) upon infection with the protozoan parasite Perkinsus olseni. MCL is synthesized in hemocytes as a approximately 74-kDa precursor and secreted into hemolymph where it is converted to 30- and 34-kDa polypeptides. The synthesis of MCL in hemocytes is stimulated by one or more factors in Perkinsus-infected hemolymph, but not directly by Perkinsus itself. MCL can bind to the surfaces of purified hypnospores and zoospores of the parasite, and this binding is inhibitable by either EDTA or GalNAc. Fluorescent beads coated with purified MCL were actively phagocytosed by hemocytes from the clam. Immunohistochemistry showed that secreted MCL is concentrated within cyst-like structures. To define the glycan binding specificity of MCL we examined its binding to an array of biotinylated glycans. MCL recognizes terminal non-reducing beta-linked GalNAc as expressed within the LacdiNAc motif GalNAcbeta1-4GlcNAcbeta1-R and glycans with terminal, non-reducing beta-linked Gal residues. Our results show that the synthesis of MCL is specifically up-regulated upon parasite infection of the clams and may serve as an opsonin through recognition of terminal GalNAc/Gal residues on the parasites.

Evolution of innate immune systems*

Innate immunity is the oldest form of defense and is found to some degree in all species. It predates the adaptive immune system, consisting of antibodies, B cells, T cells, and the major histocompatibility antigens. These are found only in higher vertebrates and have been the focus of the majority of immunological research, particularly in mice and humans, over the years. Knowledge of immunity in lower vertebrate and invertebrate species is now increasing rapidly, shedding light on the evolution of immunity and in many cases adding to our understanding of the mammalian system. Several recurring structural, genetic, and developmental mechanisms are common features in these processes in both the cellular and the molecular aspects of innate immunity.

A molecular analysis of ascidian metamorphosis reveals activation of an innate immune response

Ascidian metamorphosis represents a powerful model for comparative work on chordate development that has remained largely unexplored. We isolated transcripts differentially expressed during metamorphosis in the ascidian Boltenia villosa by suppressive PCR subtractions of staged larval and juvenile cDNAs. We employed a series of three subtractions to dissect gene expression during metamorphosis. We have isolated 132 different protein coding sequences, and 65 of these transcripts show significant matches to GenBank proteins. Some of these genes have putative functions relevant to key metamorphic events including the differentiation of smooth muscle, blood cells, heart tissue and adult nervous system from larval rudiments. In addition, a significant fraction of the differentially expressed transcripts match identified genes from the innate immune system. Innate immunity confers a rapid response to pathogen-specific molecules and/or compromised self-tissues. The activation of innate immunity genes during metamorphosis may represent the programmed maturation of the adult immune system. In addition, this immune response may be necessary for phagocytosis and re-structuring of larval tissues. An innate immune-related inflammatory response may also underlie two waves of trans-epidermal blood cell migration that occur during the swimming larval period and immediately upon settlement. We characterized these trans-epidermal migrations and discovered that some migratory cells leave the animal entirely through an anterior tunnel in the tunic. We show that these cells are positioned to detect external settlement cues and hypothesize that the innate immune system may also be employed to detect and rapidly respond to environmental settlement cues.