Molecular cloning of α-2-macroglobulin from hemocytes of common periwinkle Littorina littorea

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.

Immune repertoire in the transcriptome of Littorina littorea reveals new trends in lophotrochozoan proto-complement evolution

The evolution of complement system in invertebrates is poorly investigated. While the repertoire of complement genes in several Ecdysozoa lineages is found substantially different from that of Deuterostomia, the composition and function of the complement in the second protostome lineage, Lophotrochozoa, remains unclear. Here we report the general description of new transcriptomic data on the common periwinkle, Littorina littorea, and trace the evolutionary trajectories of the ancestral proto-complement repertoire. The repertoire is defined as immune cascade providing the minimum set of C3-associated molecules required for C3b amplification, opsonization of the targets and their phagocytosis: thioester protein (TEP) C3, serine protease C2/factor B (Bf) and complement receptors (CR). The reference transcriptome of L. littorea was built from the dual-species RNA-seq experiment with the periwinkle and its tissue digenean parasite Himasthla elongata. Five TEPs, including the ortholog of the C3, are found expressed in the in the mollusk's inflamed tissues. The homolog of the complement receptors CR1/CR2 is also expressed, however the ortholog of Bf is not. The extensive phylogenetic analysis showed that the C3 ortholog and the complement receptors are retained in all key lophotrochozoan taxa: Mollusca, Annelida and Brachiopoda. However, the Bf ortholog was lost at least three times independently in different lineages: i) Cephalopoda, ii) a common ancestor of all Gastropoda and iii) one of the Annelida lineage, Clitellata. Both C3 and Bf molecules were retained in bivalve species, brachiopods and annelid worms from the Polychaeta lineage. Hypothetically, the function of the lost Bf in these animals can be compensated by Factor L (Lf) - the serine protease first found in L. littorea and homologous to both, the Bf and the arthropod factor C (Cf). The contrast differences in proto-complement repertoire between the sister mollusk' taxa, Bivalvia and Gastropoda (the conserved and modified sets, respectively), can underlie differences in their susceptibility to digenean infection.

Comparative immunogenomics of molluscs

Comparative immunology, studying both vertebrates and invertebrates, provided the earliest descriptions of phagocytosis as a general immune mechanism. However, the large scale of animal diversity challenges all-inclusive investigations and the field of immunology has developed by mostly emphasizing study of a few vertebrate species. In addressing the lack of comprehensive understanding of animal immunity, especially that of invertebrates, comparative immunology helps toward management of invertebrates that are food sources, agricultural pests, pathogens, or transmit diseases, and helps interpret the evolution of animal immunity. Initial studies showed that the Mollusca (second largest animal phylum), and invertebrates in general, possess innate defenses but lack the lymphocytic immune system that characterizes vertebrate immunology. Recognizing the reality of both common and taxon-specific immune features, and applying up-to-date cell and molecular research capabilities, in-depth studies of a select number of bivalve and gastropod species continue to reveal novel aspects of molluscan immunity. The genomics era heralded a new stage of comparative immunology; large-scale efforts yielded an initial set of full molluscan genome sequences that is available for analyses of full complements of immune genes and regulatory sequences. Next-generation sequencing (NGS), due to lower cost and effort required, allows individual researchers to generate large sequence datasets for growing numbers of molluscs. RNAseq provides expression profiles that enable discovery of immune genes and genome sequences reveal distribution and diversity of immune factors across molluscan phylogeny. Although computational de novo sequence assembly will benefit from continued development and automated annotation may require some experimental validation, NGS is a powerful tool for comparative immunology, especially increasing coverage of the extensive molluscan diversity. To date, immunogenomics revealed new levels of complexity of molluscan defense by indicating sequence heterogeneity in individual snails and bivalves, and members of expanded immune gene families are expressed differentially to generate pathogen-specific defense responses.

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

Alpha-2 macroglobulin (α2M) is a ubiquitous protease inhibitor and considered to be an evolutionarily conserved constituent of innate host defence system. Here, an α2M gene (designated as Pfα2M) was obtained from the pearl oyster Pinctada fucata by RT-PCR, PCR walking and rapid amplification of cDNA ends (RACE). The Pfα2M cDNA consists of 6394 bp with an open reading frame (ORF) of 5745 bp encoding a protein of 1914 amino acids with a 19 residues signal peptide. Pfα2M sequence contains three putative functional domains, including a bait region, a thiol ester domain and a receptor-binding domain. Phylogenetic analysis revealed that Pfα2M is closely related to the α2Ms from other molluscs. Pfα2M was expressed in all tested tissues including digestive gland, gill, adductor muscle, mantle and foot, while the highest expression was found in hemocytes. Following challenge with Vibrio alginolyticus, Pfα2M expression in hemocytes was significantly up-regulated at 2 h and then returned to the original level at 48 h. Knockdown of Pfα2M by RNA interference significantly reduced the phagocytosis of V. alginolyticus by hemocytes in vivo, and similar results were obtained upon chemical inactivation of the reactive thioester bond in Pfα2M by methylamine treatment. Taken together, it is suggested that Pfα2M is an immune-relevant molecule and involved in phagocytosis of V. alginolyticus by P. fucata hemocytes, and the function of Pfα2M in phagocytosis is dependent on the active thioester bond.