An immunoglobulin superfamily member (CgIgIT2) functions as immune inhibitory receptor to inhibit the inflammatory cytokine expressions in Crassostrea gigas

Functional insights into immunoglobulin superfamily proteins in invertebrate neurobiology and immunity

The Immunoglobulin Superfamily (IgSF) represents a vital protein family widely distributed in animal genomes, encompassing multifunctional proteins with immunoglobulin-like domains, including immunoglobulins. These proteins play pivotal roles in various biological processes, such as development, differentiation, adhesion, activation, regulation, and signal transduction. While the functions of IgSF in vertebrates are relatively well understood, their roles in invertebrates remain underexplored. This review aims to comprehensively summarize the functions and mechanisms of IgSF in invertebrates, focusing on arthropods, mollusks, and other primitive phyla. In arthropods, research on IgSF has primarily emphasized its roles in the nervous system, especially in axonal and synaptic regulation, and its critical functions in the immune system. Studies in mollusks have predominantly highlighted the immunological functions of IgSF in pathogen recognition, clearance responses, and signal transduction. In contrast, research on protozoa and platyhelminths has mainly focused on identifying IgSF molecules, with relatively limited insights into their functional roles. In sponges, IgSF is primarily associated with cell adhesion and intercellular recognition. By exploring the genetic and protein structural diversity of IgSF in invertebrates, this review reveals their multifunctionality and complexity in biological systems. It not only enhances our understanding of the roles of IgSF in invertebrates but also lays the groundwork for future studies on their potential applications in evolutionary biology and disease models.

Molecular responses of Mytilus coruscus hemocytes to lipopolysaccharide and peptidoglycan as revealed by 4D-DIA based quantitative proteomics analysis

Mytilus are sessile filter feeders that live in close contact with numerous marine microorganisms. Hemocytes, the immunocompetent cells of Mytilus, participate in the immune response in a very efficient manner. Lipopolysaccharide (LPS) and peptidoglycan (PGN) follow specific microbe/pathogen-associated molecular patterns (MAMPs or PAMPs) and are involved in immune stimulation in host cells. This study evaluated the molecular profiles and reactions at protein level of Mytilus hemocytes after stimulation with LPS and PGN. Mytilus coruscus was challenged in vivo with LPS and PGN. The hemocytes were collected after 48 h and analyzed for quantitative proteomics, cell subpopulations, and the free amino acid composition. 4D-DIA technology-based proteomic analysis revealed different protein profiles, as well as different responses at protein level, under either the LPS or PGN challenge. C-type lectins, collagens, and CD151 protein were highly upregulated in LPS-challenged mussels, while phospholipase A2 and dCMP deaminase were highly upregulated in PGN-challenged mussels. Moreover, LPS challenge disrupted dsRNA-mediated translation and stimulated energy-related metabolism, while PGN challenge stimulated proteins involved in the inflammatory response and suppressed amino acid metabolism. In addition, the LPS and PGN challenges differed in their effects on the free amino acid composition and granulocytes ratio of the hemocytes. These findings highlight the different strategies employed by mussel hemocytes in response to different MAMPs, providing insights into the effects of LPS and PGN on Mytilus.

NCAM1-SHIP2 axis upon recognizing microbes inhibits the expressions of inflammatory factors through P38-H3K4me and P38-NF-κB pathways in oyster

Neural cell adhesion molecule 1 (NCAM1/CD56) as a well-known surface marker for natural killer (NK) cells plays important roles in cell migration, adhesion, and inflammation. In the present study, NCAM1 homolog containingthree immunoglobulin domains, one fibronectin type 3 domain, a transmembrane region and a cytoplasmic tail with two intracellular immunoreceptor tyrosine-based inhibitory motifs (ITIMs) was identified from the Pacific oyster, Crassostrea gigas (defined as CgNCAM1). The mRNA transcripts of CgNCAM1 were highly expressed in haemocytes. The mRNA expressions of CgNCAM1 in haemocytes increased significantly after Vibrio splendidus stimulation. The positive green signals of CgNCAM1 and SH2-containing inositol 5-phosphatase (CgSHIP2) could translocate onto the haemocyte membrane after V. splendidus stimulation. The recombinant extracellular domains of CgNCAM1 exhibited binding activity towards various pathogen-associated molecular patterns (PAMPs) and microbes. Upon binding to its ligands, CgNCAM1 recruited CgSHIP2 to transduce inhibitor signals to reduce the phosphorylation of CgP38. The inhibition of CgP38 reduced the methylation of histone H3K4 and nuclear translocation of NF-κB, which eventually inhibited the mRNA expressions of inflammatory factors (CgIL17-2/3/6 and CgTNF-2) to suppress inflammation. These results suggested that CgNCAM1 could function as an immune checkpoint to sense different PAMPs and microbes and reduce the inflammation through inhibiting P38-epigenetic and P38-NF-κB pathways in oysters.

Identification of a Mnlrig-1 involved in testis reproductive immunity in the oriental river prawn Macrobrachium nipponense

The testis evolves a highly organized testicular microenvironment to support spermatogenesis. However, the knowledge about it is limited in crustacean. In this study, we identified a member of immunoglobulin superfamily (IgSF) from Macrobrachium nipponense testis and explored its roles as a potential pattern recognition receptor (PRR) involved in reproductive immunity. Based on the domains it contains and homology analysis result, we designate it as leucine-rich repeats and immunoglobulin-like domains protein-1 (MnLrig-1). The Mnlrig-1 comprises a 3288 bp open reading frame (ORF) encoding a 1095 amino acid protein. MnLrig-1 is consisted of one signaling peptide; one LRR_NT domain; eight LRR domains; five LRR_TYP domains; one LRR_CT domain; three IGc2 regions; one transmembrane region, and C-terminal cytoplasmic tail, sharing similar domains with orthologs in other crustacean species. MnLrig-1 is widely expressed in various tissues of M. nipponense. Mnlrig-1 is significantly induced by LPS, PGN, Aeromonas hydrophila, and Vibrio alginolyticus challenge in the testis at 3 h and maintained a high level from 3 h to 24 h. Additionally, two recombinant immunoglobulin domains of MnLrig-1 are obtained, while only one domain shows direct binding affinity towards LPS, PGN, Escherichia coli, A. hydrophila, Staphylococcus aureus, and Bacillus subtilis in vitro. Moreover, silencing Mnlrig-1 results in a significant upregulation of three anti-lipopolysaccharide factors (ALFs) in the testis. These results reveal the potential role of MnLrig-1 as a PRR involved in the testis reproductive immunity in M. nipponense. The insights gained from this study will expand our understanding of immune system in crustacean and may have implications for aquaculture and disease management in crustaceans.

CgSHIP2 negatively regulates the mRNA expressions of CgIL-17s in response to Vibrio splendidus stimulation in Crassostrea gigas

SH2 domain containing inositol polyphosphate5-phosphatase-2 (SHIP2) is a member of the 5-phosphatase family, acting as a vital negative regulator of immune response in vertebrates. In the present study, a SHIP2 homologue (designed as CgSHIP2) was identified from Pacific oyster, Crassostrea gigas. There was a SH2 domain, an IPPc domain and a SAM domain in CgSHIP2. The mRNA transcripts of CgSHIP2 were widely expressed in all the tested tissues with the highest expression in haemolymph. The mRNA expressions of CgSHIP2 in haemocytes increased significantly at 6, 12, 48 and 72 h after Vibrio splendidus stimulation. The positive green signals of CgSHIP2 protein were mainly located in cytoplasm of haemocytes. After the expression of CgSHIP2 was inhibited by RNA interference, the mRNA transcripts of interleukin 17s (CgIL-17-1, CgIL-17-2, CgIL-17-3 and CgIL-17-6) in the haemocytes increased significantly at 24 h after V. splendidus stimulation, which were 8.15-fold (p < 0.001), 3.44-fold (p < 0.05), 2.15-fold (p < 0.01) and 4.63-fold (p < 0.05) compared with that in NC-RNAi group, respectively. Obvious branchial swelling and cilium shedding in gills were observed in CgSHIP2-RNAi group at 24 h after V. splendidus stimulation. The results suggested that CgSHIP2 played an important role in controlling inflammatory response induced by bacteria in oysters.

A DM9-containing protein from crab Eriocheir sinensis functions as a novel multipotent pattern recognition receptor

DM9-containing protein in invertebrates functions as pattern recognition receptor (PRR) to play significant roles in innate immunity. In the present study, a novel DM9-containg protein (defined as EsDM9CP-1) was identified from the Chinese mitten crab Eriocheir sinensis. EsDM9CP-1 is composed of 330 amino acids containing a Methyltransf_FA domain and two tandem DM9 repeats. The deduced amino acid sequence of EsDM9CP-1 shared low similarity with the previously identified DM9CPs from other species, and it was closely clustered with Platyhelminthes DM9CPs and then assigned into the branch of invertebrate DM9CPs in the unrooted phylogenetic tree. The mRNA transcripts of EsDM9CP-1 were highly expressed in haemocytes, gill, and heart. After Aeromonas hydrophila stimulation, the expression levels of EsDM9CP-1 mRNA in haemocytes increased significantly at 3 h (3.88-fold, p < 0.05) and 6 h (2.71-fold, p < 0.05), compared with that of PBS group, respectively. EsDM9CP-1 protein was mainly distributed in the cytoplasm and membrane of haemocytes. The recombinant EsDM9CP-1 protein (rEsDM9CP-1) exhibited binding affinity to MAN, PGN, LPS and Poly (I:C), and also to Gram-positive bacteria (Staphylococcus aureus, Micrococcus luteus and Bacillus subtilis), Gram-negative bacteria (Escherichia coli, A. hydrophila and Vibrio splendidus) and fungi (Pichia pastoris and Metschnikowia bicuspidata) in a Ca2+-dependent manner. It was able to agglutinate A. hydrophila, S. aureus, M. luteus, M. bicuspidata and P. pastoris, and inhibit the growth of A. hydrophila and M. bicuspidate. These results suggested that EsDM9CP-1 in crab not only functioned as a PRR, but also agglutinated and inhibited the growth of microbes.