Molecular cloning and expression analysis of a new lily-type lectin in the rock bream, Oplegnathus fasciatus

Immunological characterization and function analysis of L-type lectin from spotted knifejaw, Oplegnathus punctatus

Lily-type lectin (LTL) plays significant roles in innate immune response against pathogen infection. LTL in animals and plants has received widespread attention. In the present study, an LTL (OppLTL) was identified from spotted knifejaw Oplegnathus punctatus. The OppLTL encoded a typical Ca²⁺-dependent carbohydrate-binding protein containing a CRD domain. The qRT-PCR showed that it was mainly expressed in the gill and was significantly upregulated after Vibrio anguillarum challenge. The agglutination analysis showed that the recombinant OppLTL could bind and agglutinate Gram-negative and Gram-positive bacteria in a Ca²⁺-dependent manner. However, the binding activity was different. Meanwhile, the recombinant OppLTL could hemagglutinate mammalian and teleost erythrocytes. Subcellular localization revealed that OppLTL was mainly detected in the cytoplasm of HEK293T cells. The dual-luciferase analysis revealed that OppLTL could inhibit the activity of the NF-κB signal pathway in HEK293T cells after OppLTL overexpression. These findings collectively demonstrated that OppLTL could be involved in host innate immune response and defense against bacterial infection in spotted knifejaw.

Lectin-Like Bacteriocins

Bacteria produce a diverse array of antagonistic compounds to restrict growth of microbial rivals. Contributing to this warfare are bacteriocins: secreted antibacterial peptides, proteins and multi-protein complexes. These compounds typically eliminate competitors closely related to the producer. Lectin-like bacteriocins (LlpAs) constitute a distinct class of such proteins, produced by Pseudomonas as well as some other proteobacterial genera. LlpAs share a common architecture consisting of two B-lectin domains, followed by a short carboxy-terminal extension. Two surface-exposed moieties on susceptible Pseudomonas cells are targeted by the respective lectin modules. The carboxy-terminal domain binds D-rhamnose residues present in the lipopolysaccharide layer, whereas the amino-terminal domain interacts with a polymorphic external loop of the outer-membrane protein insertase BamA, hence determining selectivity. The absence of a toxin-immunity module as found in modular bacteriocins and other polymorphic toxin systems, hints toward a novel mode of killing initiated at the cellular surface, not requiring bacteriocin import. Despite significant progress in understanding the function of LlpAs, outstanding questions include the secretion machinery recruited by lectin-like bacteriocins for their release, as well as a better understanding of the environmental signals initiating their expression.

Identification and characterization of a calcium-dependent lily-type lectin from black rockfish (Sebastes schlegelii): Molecular antennas are involved in host defense via pathogen recognition

Lily-type lectins (LTLs) are soluble pathogen recognition receptors (PRRs) that contain one or more characteristic carbohydrate recognition domains (CRDs), through which LTLs bind reversibly and specifically to cognate sugar moieties present on the invading pathogen membrane and trigger the host innate immune responses. In this study, we identified a LTL homolog (SsLTL) from black rockfish (Sebastes schlegelii) transcriptome database and its functional roles in innate immunity was investigated in vitro and in vivo. Three mannose-binding sites were found in the protein sequence of SsLTL, among which two sites are conserved with those in mannose-binding lectins of monocotyledonous plants. SsLTL were highly expressed in both the external and internal mucosal tissues of healthy rockfish. During the immune challenge, early up-regulation of SsLTL mRNA expression showed in gill and blood upon both poly I:C and S. iniae challenges. In contrast, the challenge with lipopolysaccharide significantly down-regulated SsLTL expression in both examined tissues. Recombinant SsLTL showed a hemagglutination activity toward fish erythrocytes, which could be enhanced by the addition of calcium ions. Furthermore, strong agglutination activity of SsLTL was also observed with a broad range of fish pathogenic bacteria. Our results implied the crucial role of SsLTL as a PRR molecule in the black rockfish defense mechanism against invading microbial pathogens.

Expression Analysis of Lily Type Lectin Isotypes in the Rock Bream, Oplegnathus fasciatus: in the Tissue, Developmental Stage and Viral Infection

Lectins belong to the pattern-recognition receptors (PRRs) class and play important roles in the recognition and elimination of pathogens via the innate immune system. Recently, it was reported that lily-type lectin-1 is involved when a pathogen attacks in the early immune response of fish. However, this study is limited to information that the lectin is involved in the innate immune response against viral infection. In the present study, the lily-type lectin-2 and -3 of Oplegnathus fasciatus (OfLTL-2 and 3) have been presented to be included B-lectin domain and two D-mannose binding sites in the amino acid sequence that an important feature for the fundamental structure. To investigate the functional properties of OfLTLs, the tissue distribution in the healthy rock bream and temporal expression during early developmental stage analysis are performed using quantitative real-time PCR. OfLTL-2 and 3 are predominantly expressed in the liver and skin, but rarely expressed in other organ. Also, the transcripts of OfLTLs are not expressed during the early developmental stage but its transcripts are increased after immune-related organs which are fully formed. In the challenge experiment with RBIV (rock bream iridovirus), the expression of OfLTLs was increased much more strongly in the late response than the early, unlike previously known. These results suggest that OfLTLs are specifically expressed in the immune-related tissues when those organs are fully formed and it can be inferred that the more intensively involved in the second half to the virus infection.