Identification, expression pattern and potential role of variable lymphocyte receptor Aj-VLRA from Apostichopus japonicus in response to bacterial challenge

LvCD14L Acts as a Novel Pattern Recognition Receptor and a Regulator of the Toll Signaling Pathway in Shrimp

Leucine-rich repeat (LRR) is a structural motif has important recognition function in immune receptors, such as Tolls and NOD-like receptors (NLRs). The immune-related LRR proteins can be divided into two categories, LRR-containing proteins and LRR-only proteins. The latter contain LRR motifs while they are without other functional domains. However, the functional mechanisms of the LRR-only proteins were still unclear in invertebrates. Here, we identified a gene encoding a secretory LRR-only protein, which possessed similarity with vertebrate CD14 and was designated as LvCD14L, from the Pacific whiteleg shrimp Litopenaeus vannamei. Its transcripts in shrimp hemocytes were apparently responsive to the infection of Vibrio parahaemolyticus. Knockdown of LvCD14L with dsRNA resulted in significant increase of the viable bacteria in the hepatopancreas of shrimp upon V. parahaemolyticus infection. Further functional studies revealed that LvCD14L could bind to microorganisms' PAMPs, showed interaction with LvToll1 and LvToll2, and regulated the expression of LvDorsal and LvALF2 in hemocytes. These results suggest that LvCD14L functions as a pattern recognition receptor and activates the NF-κB pathway through interaction with LvTolls. The present study reveals a shrimp LvCD14L-Tolls-NF-κB signaling pathway like the CD14/TLR4/NF-κB signaling pathway in mammalians, which enriches the functional mechanism of secretory LRR-only immune receptors during pathogens infection in invertebrates.

Alternative splicing derived invertebrate variable lymphocyte receptor displays diversity and specificity in immune system of crab Eriocheir sinensis

Variable lymphocyte receptors (VLRs) play vital roles in adaptive immune system of agnathan vertebrate. In the present study, we first discover a novel VLR gene, VLR2, from an invertebrate, the Chinese mitten crab, Eriocheir sinensis. VLR2 has ten different isoforms formed via alternative splicing, which is different from that in agnathan vertebrate with the assembly of LRR modules. The longest isoform, VLR2-L, responds to Gram-positive bacteria Staphylococcus aureus challenge specifically, while shows no response to Gram-negative bacteria Vibrio parahaemolyticus challenge, confirmed by recombinant expression and bacterial binding experiments. Interestingly, VLR2s with short LRRs regions (VLR2-S8 and VLR2-S9) tend to bind to Gram-negative bacteria rather than Gram-positive bacteria. Antibacterial activity assay proves six isoforms of VLR2 have pluralistic antibacterial effects on bacteria which were never reported in invertebrate. These results suggest that the diversity and specificity of VLR2 resulted from alternative splicing and the length of the LRRs region. This pathogen-binding receptor diversity will lay the foundation for the study of immune priming. Furthermore, studying the immune function of VLR2 will provide a new insight into the disease control strategy of crustacean culture.

New insight of variable lymphocyte receptor-likes in anti-bacteria activity from Eriocheir sinensis

The Chinese mitten crab, Eriocheir sinensis, is a vital freshwater aquaculture species in China, however, is also facing various crab disease threats. In the present study, we identify three novel variable lymphocyte receptor-like (VLR-like) genes-VLR-like1, VLR-like3 and VLR-like4-from E. sinensis, which play vital roles in adaptive immune system of agnathan vertebrates. The bacterial challenge, bacterial binding and antibacterial-activity experiments were applied to study immune functions of VLR-likes, and the transcriptomic data from previous E. sinensis bacterial challenge experiments were analyzed to speculate the possible signaling pathway. VLR-like1 and VLR-like4 can respond to Staphylococcus aureus challenge and inhibit S. aureus specifically. VLR-like1 and VLR-like4 possess broad-spectrum bacteria-binding ability whereas VLR-like3 do not. VLR-likes in E. sinensis could associate with the Toll-like receptor (TLR) signaling pathway. The above results suggest that VLR-likes defend against bacteria invasion though exerting anti-bacteria activity, and probably connect with the TLR signaling pathway. Furthermore, studying the immune functions of these VLR-likes will provide a new insight into the disease control strategy of crustacean culture.

Identification and functional study of an LRR domain containing membrane protein in Litopenaeus vannamei

Leucine-rich repeat (LRR) is a vital structure in some pattern recognition receptors such as TLRs, NLRs and newly reported LRR-containing proteins. Apart from some limited reported LRR-containing proteins, most of LRR proteins, especially immune-related proteins, remain uncharacterized functionally. In the present study, a transmembrane protein containing several LRR motifs, designated as LvLRRm, was identified from the shrimp Litopenaeus vannamei. LvLRRm contained a long signal peptide, one LRRNT region, 12 LRR motifs, one LRRCT region and a transmembrane region. The transcripts of LvLRRm were widely distributed in all tested tissues of shrimp and they were responsive to Vibrio parahaemolyticus infection in several immune-related tissues including Oka, intestine, gill and hemocytes. Knockdown of LvLRRm by dsRNA interference led to a decreased survival rate of shrimp infected by Vibrio parahaemolyticus and an increased in vivo Vibrio propagation. Meanwhile, knockdown of LvLRRm also down-regulated the expression levels of genes involved in antibacterial immune signaling pathways, including the transcription factors LvDorsal and LvRelish, and several antimicrobial peptides. These data suggested that LvLRRm played important roles in shrimp against Vibrio infection, which was probably functioning through activation of antibacterial immune signaling pathways. The present study provided new evidence to elucidate the immune function of LRR-containing proteins in invertebrates.

Identification, functional characterization and the potential role of variable lymphocyte receptor EsVLRA from Eriocheir sinensis in response to secondary challenge after Vibrio parahaemolyticus vaccine

Variable lymphocyte receptors (VLRs) play an important role via their antigen-special reorganization in jawless vertebrates (agnathans) adaptive immune response. In the present study, the open reading frame (ORF) of Eriocheir sinensis VLRA (designated as EsVLRA) was identified. EsVLRA comprised a 799-amino-acid polypeptide with one LRR_NT domain, thirteen LRR domains and one LRR_CT domain, which showed a high domain consistency of the VLR genes in lamprey (Petromyzon marinus). The transcript of EsVLRA was detected in all examined tissues with the highest level detected in hepatopancreas. Notably, the expression of EsVLRA in hepatopancreas, gonads, gill and intestine of male crabs was significantly higher than that in females. The recombinant EsVLRA exhibited strong bacteria-binding activity rather than antibacterial activity, suggesting its crucial role in immune recognition. Furthermore, 6 h earlier response and a significantly higher peak of EsVLRA mRNA expression was observed after challenge with live Vibrio parahaemolyticus (240.6-fold, P < 0.01, crabs receive secondary challenge after V. parahaemolyticus vaccine to the carbs only receive twice PBS injection, N = 6), compared with those only received first injection with formalin-inactivated V. parahaemolyticus (39.7-fold, P < 0.01, challenge 6 h to vaccination 12 h). The findings of this study together demonstrated that EsVLRA plays an important role in the immune system of E. sinensis, serving as a pattern recognition receptor and involving in the immune priming.

A novel LRR-only protein mediates bacterial proliferation in hemolymph through regulating expression of antimicrobial peptides in mollusk Chlamys farreri

Leucine-rich repeat (LRR)-only proteins are involved in innate immune responses through mediating protein-ligand or protein-protein interactions, yet the exact roles of most LRR-only proteins in invertebrates are not well documented. In the present study, a novel LRR-only protein (designated CfLRRop-7) was identified in Zhikong scallop Chlamys farreri. The full-length cDNA sequence of CfLRRop-7 was 1463 bp and contained an open reading frame of 1086 bp, which encoded a protein of 361 amino acids. Five LRR motifs with a conserved signature sequence LxxLxLxxNxL were identified in the predicted protein sequence. The expression of CfLRRop-7 was particularly high in hemocytes. The expression of CfLRRop-7 was relatively high in oocytes and embryos during the ontogenesis of scallops. CfLRRop-7 expression changed in hemocytes in response to stimulation with different microbes, including Vibrio splendidus, Staphylococcus aureus and Pichia pastoris. CfLRRop-7 recognized five kinds of ligands/agonists. CfLRRop-7 recombinant protein inhibited bacterial proliferation in hemolymph and induced lysozyme activity in serum. After knocking down CfLRRop-7, the mRNA expression of selected antimicrobial peptides was reduced. All these results indicated that CfLRRop-7 might be a potential pattern recognition receptor that recognizes various pathogen associated molecular patterns, and regulates antibacterial immunity in scallops.

The various components implied the diversified Toll-like receptor (TLR) signaling pathway in mollusk Chlamys farreri

Toll-like receptor (TLR) signaling pathway, composed of various components, plays pivotal roles in host innate immune defense mechanism. In the present study, twenty-nine TLR signaling pathway components, including receptors, adaptors, transduction molecules and immune effectors, were identified in Zhikong scallop Chlamys farreri via assembling and screening public available transcriptomic data and expression sequence tags (ESTs). These identified TLR signaling pathway components were constitutively expressed and detectable in various tissues, and almost all of them were highly expressed in gill and hepatopancreas. These results indicated the presence of TLR signaling pathways in both MyD88-dependent and MyD88-independent forms in scallop, and implied the diversified TLR signaling pathway in mollusk C. farreri.

The versatile functions of LRR-only proteins in mollusk Chlamys farreri

Leucine-rich repeat (LRR)-only proteins are involved in the innate immune responses as they mediate protein-ligand interactions. In the present study, three novel LRR-only proteins, CfLRRop-4, CfLRRop-5 and CfLRRop-6, were identified and characterized from Zhikong scallop Chlamys farreri. They all contained LRR motifs with consensus signature sequences of LxxLxLxxNxL or LxxLxLxxCxxL. All the mRNA transcripts of three CfLRRops were high abundant in hepatopancreas, gills and gonads, and their mRNA transcripts in hemocytes could respond to the stimulations of different microbes, including Vibrio anguillarum, Micrococcus luteus and Pichia pastoris. These three CfLRRops exhibited similar ligand binding and recognition characteristics as Toll-like receptors (TLRs) and NOD-like receptors (NLRs). The immune effectors, including tumor necrosis factor α, superoxide dismutase, catalase and lysozyme, varied significantly after the scallops were stimulated by recombinant LRR-only proteins. All these results indicated that LRR-only proteins are functionally differentiated and exhibit different immunomodulation activities on various downstream immune effectors.

Two novel LRR-only proteins in Chlamys farreri: Similar in structure, yet different in expression profile and pattern recognition

Leucine-rich repeat (LRR)-only proteins could mediate protein-ligand and protein-protein interactions and be involved in the immune response. In the present study, two novel LRR-only proteins, CfLRRop-2 and CfLRRop-3, were identified and characterized from scallop Chlamys farreri. They both contained nine LRR motifs with the consensus signature sequence LxxLxLxxNxL and formed typical horseshoe structure. The CfLRRop-2 and CfLRRop-3 mRNA transcripts were constitutively expressed in haemocytes, muscle, mantle, gill, haepatopancreas and gonad, with the highest expression level in haepatopancreas and gill, respectively. During the ontogenesis of scallop, the mRNA transcripts of CfLRRop-2 were kept at a high level in oocytes and embryos, while those of CfLRRop-3 were expressed at a rather low level from oocytes to blastula. Their mRNA transcripts were significantly increased after the stimulation of lipopolysaccharide (LPS), peptidoglycan (PGN), glucan (GLU) and polyinosinic-polycytidylic acid (poly I:C), and the mRNA expression of CfLRRop-2 rose more intensely than that of CfLRRop-3. After the suppression of CfTLR (previously identified Toll-like receptor in C. farreri) via RNA interference (RNAi), CfLRRop-3 mRNA transcripts increased more intensely and lastingly than those of CfLRRop-2. The rCfLRRop-3 protein could bind LPS, PGN, GLU and poly I:C, while rCfLRRop-2 exhibited no significant binding activity to them. Additionally, rCfLRRop-2 could significantly induce the release of TNF-α from the mixed primary cultured scallop haemocytes, but rCfLRRop-3 failed. These results collectively indicated that CfLRRop-2 might act as an immune effector or pro-inflammatory factor, while CfLRRop-3 would function as a pattern recognition receptor (PRR), suggesting the function of LRR-only protein family has differentiated in scallop.