An alpha-2 macroglobulin in the pearl oyster Pinctada fucata: Characterization and function in hemocyte phagocytosis of Vibrio alginolyticus

A C1q domain-containing protein from Scapharca subcrenata may participate in the immune defense against pathogen infection

The C1q domain-containing proteins (C1qDCs) have been shown to play crucial roles in immune responses among invertebrates, but few studies have focused on immune regulatory pathways in bivalves. In this study, we identified a C1qDC gene with a typical C1q domain (designated as SsC1qDC1) from Scapharca subcrenata and characterized its potential immune response against pathogen infection. The full-length cDNA sequence of SsC1qDC1 was 870 bp, encoding a peptide of 259 amino acids with an N-terminal signal peptide. Sequence analysis indicated that SsC1qDC1 was not conserved compared with C1qDCs from other bivalves. SsC1qDC1 was detected in all examined tissues, with higher expression levels in hepatopancreas, mantle, and gill. Concurrently, the expressions of SsC1qDC1 increased significantly from eggs to the gastrula stage and increased remarkably following Vibrio parahaemolyticus challenge. RNA interference-mediated knockdown of SsC1qDC1 significantly repressed the expressions of maternally derived immune-related genes (Vg, Lysozyme, Dscam, TEP, complement C3, and C1qDC3), while remarkably augmenting the expression of phenoloxidase activation factor PAF3. Furthermore, compared to the controls, the phagocytosis rate and total number of hemocytes were markedly reduced in the SsC1qDC1-silenced ark shells post V. parahaemolyticus infection, and thus, a significant increase in mortality rate and hemocyte apoptotic levels. Collectively, these findings implied that SsC1qDC1 may play crucial roles in immune defense against pathogen infection in ark shells.

Comprehensive metabolomics and transcriptomics analyses investigating the regulatory effects of different sources of dietary astaxanthin on the antioxidant and immune functions of commercial-sized rainbow trout

Astaxanthin is an important aquatic feed additive that enhances the antioxidant capacity, and immune function of rainbow trout (Oncorhynchus mykiss); however, very limited information is available on its underlying molecular mechanisms. Haematococcus pluvialis powder, Phaffia rhodozyma powder, and synthetic astaxanthin were added to the commercial feed (no astaxanthin, NA) to prepare three experimental feeds, referred to as the HPA, PRA, and SA groups, respectively, and their actual astaxanthin contents were 31.25, 32.96, and 31.50 mg.kg-1, respectively. A 16-week feeding trial was conducted on the O. mykiss with an initial body weight of 669.88 ± 36.22 g. Serum and head kidney samples from commercial-sized O. mykiss were collected for metabolomics and transcriptomics analysis, respectively. Metabolomics analysis of the serum revealed a total of 85 differential metabolites between the astaxanthin-supplemented group and the control group. These metabolites were involved in more than 30 metabolic pathways, such as glycerophospholipid metabolism, fatty acid biosynthesis, linoleic acid metabolism, and arginine and proline metabolism. It is speculated that different sources of dietary astaxanthin may regulate antioxidant capacity and immunity mainly by affecting lipid metabolism and amino acid metabolism. Transcriptomic analysis of the head kidney revealed that the differentially expressed genes between the astaxanthin-supplemented group and the control group, such as integrin beta-1 (ITGB1), alpha-2-macroglobulin (A2M), diamine acetyltransferase 1 (SAT1), CCAAT/enhancer-binding protein beta (CEBPB) and DNA damage-inducible protein 45 alpha (GADD45A), which are involved in cell adhesion molecules, the FoxO signaling pathway, phagosomes, and arginine and proline metabolism and play regulatory roles in different stages of the antioxidant and immune response of O. mykiss.

Transcriptomic profiling of immune-associated molecules in the coelomocytes of lugworm Arenicola marina (Linnaeus, 1758)

Organization and functioning of immune system remain unevenly studied in different taxa of lophotrochozoan animals. We analyzed transcriptomic data on coelomocytes of the lugworm Arenicola marina (Linnaeus, 1758; Annelida, Polychaeta) to gain insights into the molecular mechanisms involved in polychaete immunity. Coelomocytes are specialized motile cells populating coelomic fluid of annelids, responsible for cellular defense reactions and providing humoral immune factors. The transcriptome was enriched with immune-related transcripts by challenging the cells in vitro with lipopolysaccharides of Escherichia coli and Zymosan from Saccharomyces cerevisiae. Our analysis revealed a multifaceted and complex internal defense system of the lugworm. A. marina possesses orthologs of proto-complement-like factors: six thioester-containing proteins, a complement-like receptor, and a MASP-related serine protease (MReM2). A. marina coelomocytes employ pattern-recognition receptors to detect pathogens and regulate immune responses. Among them, there are 18 Toll-like receptors and various putative lectin-like proteins with evolutionary conserved and taxa-specific domains. C-type lectins and a novel family of Gal-binding and CUB domains containing receptors were the most abundant in the transcriptome. The array of pore-forming proteins in the coelomocytes was surprisingly reduced compared to that of other invertebrate species. We characterized a set of conserved proteins metabolizing reactive oxygen species and nitric oxide and expanded the arsenal of potential antimicrobial peptides. Phenoloxidase activity in immune cells of lugworm is mediated only by laccase enzyme. The described repertoire of immune-associated molecules provides valuable candidates for further functional and comparative research on the immunity of annelids.

Hemolymph proteins: An overview across marine arthropods and molluscs

In this compilation we collect information about the main protein components in hemolymph and stress the continued interest in their study. The reasons for such an attention span several areas of biological, veterinarian and medical applications: from the notions for better dealing with the species - belonging to phylum Arthropoda, subphylum Crustacea, and to phylum Mollusca - of economic interest, to the development of 'marine drugs' from the peptides that, in invertebrates, act as antimicrobial, antifungal, antiprotozoal, and/or antiviral agents. Overall, the topic most often on focus is that of innate immunity operated by classes of pattern-recognition proteins. SIGNIFICANCE: The immune response in invertebrates relies on innate rather than on adaptive/acquired effectors. At a difference from the soluble and membrane-bound immunoglobulins and receptors in vertebrates, the antimicrobial, antifungal, antiprotozoal and/or antiviral agents in invertebrates interact with non-self material by targeting some common (rather than some highly specific) structural motifs. Developing this paradigm into (semi) synthetic pharmaceuticals, possibly optimized through the modeling opportunities offered by computational biochemistry, is one of the lessons today's science may learn from the study of marine invertebrates, and specifically of the proteins and peptides in their hemolymph.

Molecular Characterization of Complement Component 3 (C3) in the Pearl Oyster Pinctada fucata Improves Our Understanding of the Primitive Complement System in Bivalve

As the central component in the complement system, complement component 3 (C3) plays essential roles in both the innate and adaptive immune responses. Here, a C3 gene (designated as pf-C3) was obtained from the pearl oyster Pinctada fucata by RT-PCR and rapid amplification of cDNA ends (RACE). The pf-C3 cDNA consists of 5,634 bp with an open reading frame (ORF) of 5,193 bp encoding a protein of 1,730 amino acids with a 19 residue signal peptide. The deduced pf-C3 protein possessed the characteristic structural features present in its homologs and contained the A2M_N_2, ANATO, A2M, A2M_comp, A2M_recep, and C345C domains, as well as the C3 convertase cleavage site, thioester motif, and conserved Cys, His, and Glu residues. Phylogenetic analysis revealed that pf-C3 is closely related to the C3s from other mollusks. Pf-C3 mRNA was expressed in all examined tissues including gill, digestive gland, adductor muscle, mantle and foot, while the highest expression was found in the digestive gland. Following the challenge with Vibrio alginolyticus, pf-C3 expression was significantly induced in hemocytes. Luciferase reporter assays indicated that pf-C3a could activate the NF-κB signal pathway in HEK293T cells. Further knockdown of pf-C3 by specific siRNA could significantly reduce the phagocytosis of V. alginolyticus by hemocytes in vitro. These results would help increase understanding of the function of C3 in the invertebrate immune system and therefore provide new insights into the roles of the primitive complement system in invertebrates.

Survival, retention rate and immunity of the black shell colored stocks of pearl oyster Pinctada fucata martensii after grafting operation

We have developed a black shell colored selected line observed to have higher survival ability. In this study, to understand its immune capacity, total carotenoid content (TCC) of the black shell colored line (BG) and the control group (CG) were compared. Survival and retention rates, immunity and antioxidant capacity of BG were compared relative to CG at different times after grafting operation. The results showed that BG had significantly larger TCC than CG (P < 0.05). BG had significantly higher survival and retention rates than CG on days 7, 30 and 360 after grafting (P < 0.05). On days 360, BG had significantly larger pearl thickness than CG (P < 0.05). BG exhibited increased ACP, AKP, SOD, CAT, TAOC and LZ activity than the CG on 0 h, 12 h, 1 d, 3 d, 5 d, 7 d and 30 d after grafting. BG had higher expression levels of Fascin, SOD, CDK-7, CDAP-1, IRAK-1, α2m, GST-1, TRAF-3 and Caspase-2 than CG. The results suggested that BG had higher immune competence and pearl production performances, which is promising to improve pearl quality and production.

Differential proteomic profiles and characterizations between hyalinocytes and granulocytes in ivory shell Babylonia areolata

The haemocytes of the ivory shell, Babylonia areolata are classified by morphologic observation into the following types: hyalinocytes (H) and granulocytes (G). Haemocytes comprise diverse cell types with morphological and functional heterogene and play indispensable roles in immunological homeostasis of invertebrates. In the present study, two types of haemocytes were morphologically identified and separated as H and G by Percoll density gradient centrifugation. The differentially expressed proteins were investigated between H and G using mass spectrometry. The results showed that total quantitative proteins between H and G samples were 1644, the number of up-regulated proteins in G was 215, and the number of down-regulated proteins in G was 378. Among them, cathepsin, p38 MAPK, toll-interacting protein-like and beta-adrenergic receptor kinase 2-like were up-regulated in G; alpha-2-macroglobulin-like protein, C-type lectin, galectin-2-1, galectin-3, β-1,3-glucan-binding protein, ferritin, mega-hemocyanin, mucin-17-like, mucin-5AC-like and catalytic subunit of protein kinase A were down-regulated in G. The results showed that the most significantly enriched KEGG pathways were the pathways related to ribosome, phagosome, endocytosis, carbon metabolism, protein processing in endoplasmic reticulum and oxidative phosphorylation. For phagosome and endocytosis pathway, the number of down-regulation proteins in G was more than that of up-regulation proteins. For lysosome pathway, the number of up-regulation proteins in G was more than that of down-regulation proteins. These results suggested that two sub-population haemocytes perform the different immune functions in B. areolata.

Two alpha-2 macroglobulin from Portunus trituberculatus involved in the prophenoloxidase system, phagocytosis and regulation of antimicrobial peptides

Alpha-2 macroglobulin (A2M) is a ubiquitous protease inhibitor involved in the innate host defense system. Herein, two distinct A2M genes (designated as PtA2M-1 and PtA2M-2, respectively) were isolated from the swimming crab Portunus trituberculatus. PtA2M-1 and PtA2M-2 encoded proteins with 1541 or 1516 amino acids, respectively, containing the typically functional domains of A2M. Unlike highly expressed in hemocytes of most arthropods, PtA2M-1 and PtA2M-2 were predominantly detected in gill, eyestalk and digestive tracks. During the embryonic stages, PtA2Ms were found to be expressed most highly in fertilized eggs, suggesting their maternal origin. After challenged with Vibrio alginolyticus, the transcripts of PtA2Ms showed similar time-dependent response expression pattern, while PtA2M-1 was more sensitive to Micrococcus luteus and Pichia pastoris infection than PtA2M-2. Knockdown of PtA2M-1 or PtA2M-2 could significantly enhance the expression of prophenoloxidase (proPO) associated genes (PtproPO and PtPPAF) and serine protease related genes (PtcSP1-3 and PtSPH), however, PtLSZ and the phagocytosis-related genes (PtMyosin and PtRab5) were effectively inhibited. These results were further supported by the PO and lysozyme activities in hemolymph of the PtA2M-1- or PtA2M-2-silenced crabs. In addition, PtA2M-1 and PtA2M-2 could regulate the expression of antimicrobial peptide (AMP) genes (PtALF1-3, PtCrustin1 and PtCrustin3) through the Toll and NF-κB pathways. Our findings together suggest that PtA2Ms might function in crab host defense via regulating the proPO system, phagocytosis and the expression of AMP genes.

The immune system of the freshwater zebra mussel, Dreissena polymorpha, decrypted by proteogenomics of hemocytes and plasma compartments

The immune system of bivalves is of great interest since it reflects the health status of these organisms during stressful conditions. While immune molecular responses are well documented for marine bivalves, few information is available for continental bivalves such as the zebra mussel, Dreissena polymorpha. A proteogenomic approach was conducted on both hemocytes and plasma to identified immune proteins of this non-model species. Combining transcriptomic sequences with mass spectrometry data acquired on proteins is a relevant strategy since 3020 proteins were identified, representing the largest protein inventory for this sentinel organism. Functional annotation and gene ontology (GO) analysis performed on the identified proteins described the main molecular players of hemocytes and plasma in immunity. GO analysis highlights the complementary immune functions of these two compartments in the management of micro-organisms. Functional annotation revealed new mechanisms in the immune defence of the zebra mussel. Proteins rarely observed in the hemolymph of bivalves were pinpointed such as natterin-like and thaumatin-like proteins. Furthermore, the high abundance of complement-related proteins observed in plasma suggested a strong implication of the complement system in the immune defence of D. polymorpha. This work brings a better understanding of the molecular mechanisms involved in zebra mussel immunity. SIGNIFICANCE: Although the molecular mechanisms of marine bivalves are widely investigated, little information is known for continental bivalves. Moreover, few proteomic studies described the complementarity of both hemolymphatic compartments (cellular and plasmatic) in the immune defence of invertebrates. The recent proteogenomics concept made it possible to discover proteins in non-model organisms. Here, we propose a proteogenomic strategy with the zebra mussel, a key sentinel species for biomonitoring of freshwater, to identify and describe the molecular actors involved in the immune system in both hemocytes and plasma compartments. More widely, this study provided new insight into bivalve immunity.