Novel carbohydrate-recognition mode of the invertebrate C-type lectin SPL-1 fromSaxidomus purpuratusrevealed by the GlcNAc-complex crystal in the presence of Ca2+

A c-type lectin HcLec1 with dual function of immunology and mineralization from the freshwater oyster (Hyriopsis cumingii Lea)

Background

Shell and pearl formation in bivalves is a sophisticated biomineralization process that encompasses immunological and mineralization aspects, particularly during shell repair and the initial stages of pearl cultivation when a nucleus is inserted. Here, we describe a novel C-type lectin, HcLec1, isolated and characterized from the freshwater pearl mussel Hyriopsis cumingii Lea.

Methods

Immune challenge, RNA interference (RNAi) experiments, ELISA, and antibacterial assays were employed to investigate the role of HcLec1 in innate immunity. We also established shell damage repair and pearl nucleus insertion models to examine the impact of HcLec1 on the biomineralization process in Hyriopsis cumingii Lea. In vitro calcium carbonate crystallization assays were conducted to explore the direct role of HcLec1 in calcium carbonate crystal formation.

Results

The HcLec1 gene sequence is a full-length cDNA of 1552 bp, encoding 240 amino acids. HcLec1 comprises an N-terminal signal peptide and a carbohydrate-recognition domain (CRD), with QPD (Gln-Pro-Asp) and MND (Met-Asn-Asp) motifs for polysaccharide binding. Tissue expression analysis showed that HcLec1 is predominantly expressed in the gill tissue of Hyriopsis cumingii Lea under normal conditions, and its expression is significantly elevated in both gill and pearl sac tissues following nucleus insertion for pearl cultivation (P < 0.05). After immune stimulation with Aeromonas hydrophila and lipopolysaccharides (LPS), HcLec1 expression levels significantly increased in both cases (P < 0.01), indicating a role in bivalve innate immunity. RNA interference (RNAi)-mediated knockdown of HcLec1 led to a significant decrease in the expression levels of immune-related genes (WAP, α2m, and Lyso) and mineralization-related genes (CA, CHS, Nacrein, and Pif) (P < 0.05). In animal models for shell damage and nucleus insertion in pearl cultivation, HcLec1 showed a consistent expression pattern, with an initial significant decrease followed by a marked increase, peaking at day 14 (P < 0.05). This suggests a role for HcLec1 in pearl formation and shell repair. The recombinant HcLec1 protein demonstrated binding affinity to LPS and PGN, a robust ability to agglutinate Escherichia coli, Staphylococcus aureus, Aeromonas veronii, and Aeromonas hydrophila, and significantly inhibited bacterial growth (P < 0.05). Moreover, rHcLec1 promoted calcite crystal formation in saturated calcium carbonate solutions and altered crystal morphology.

Discussion

The HcLec1 gene plays a pivotal role in both innate immunity and biomineralization in the triangle sail mussel. This study enhances our understanding of the functional diversity of C-type lectins and provides a foundation for future studies on shell repair and pearl growth.

Functional Diversity of Novel Lectins with Unique Structural Features in Marine Animals

Due to their remarkable structural diversity, glycans play important roles as recognition molecules on cell surfaces of living organisms. Carbohydrates exist in numerous isomeric forms and can adopt diverse structures through various branching patterns. Despite their relatively small molecular weights, they exhibit extensive structural diversity. On the other hand, lectins, also known as carbohydrate-binding proteins, not only recognize and bind to the diverse structures of glycans but also induce various biological reactions based on structural differences. Initially discovered as hemagglutinins in plant seeds, lectins have been found to play significant roles in cell recognition processes in higher vertebrates. However, our understanding of lectins in marine animals, particularly marine invertebrates, remains limited. Recent studies have revealed that marine animals possess novel lectins with unique structures and glycan recognition mechanisms not observed in known lectins. Of particular interest is their role as pattern recognition molecules in the innate immune system, where they recognize the glycan structures of pathogens. Furthermore, lectins serve as toxins for self-defense against foreign enemies. Recent discoveries have identified various pore-forming proteins containing lectin domains in fish venoms and skins. These proteins utilize lectin domains to bind target cells, triggering oligomerization and pore formation in the cell membrane. These findings have spurred research into the new functions of lectins and lectin domains. In this review, we present recent findings on the diverse structures and functions of lectins in marine animals.

C-type lectin-(like) fold - Protein-protein interaction patterns and utilization

The C-type lectin-like fold (CTL fold) is a building block of many proteins, including saccharide-binding lectins, natural killer cell receptors, macrophage mannose receptor, selectins, collectins, snake venoms and others. Some are important players in innate immunity and are involved in the first-line response to virally infected cells or cancer cells, some play a role in antimicrobial defense, and some are potential targets for fight against problems connected with allergies, obesity, and autoimmunity. The structure of a CTL domain typically contains two α-helices, two small β-sheets and a long surface loop, with two or three disulfide bridges stabilizing the structure. This small domain is often involved in interactions with a target molecule, however, utilizing varied parts of the domain surface, with or without structural modifications. More than 500 three-dimensional structures of CTL fold-containing proteins are available in the Protein Data Bank, including a significant number of complexes with their key interacting partners (protein:protein complexes). The amount of available structural data enables a detailed analysis of the rules of interaction patterns utilized in activation, inhibition, attachment and other pathways or functionalities. Interpretation of known CTL receptor structures and all other CTL-containing proteins and complexes with described three-dimensional structures, complemented with sequence/structure/interaction correlation analysis offers a comprehensive view of the rules of interaction patterns of the CTL fold. The results are of value for prediction of interaction behavior of so far not understood CTL-containing proteins and development of new protein binders based on this fold, with applications in biomedicine or biotechnologies. It follows from the available structural data that almost the whole surface of the CTL fold is utilized in protein:protein interactions, with the heaviest frequency of utilization in the canonical interaction region. The individual categories of interactions differ in the interface buildup strategy. The strongest CTL binders rely on interfaces with large interaction area, presence of hydrophobic core, or high surface complementarity. The typical interaction surfaces of the fold are not conserved in amino acid sequence and can be utilized in design of new binders for biotechnological applications.