Lectin-like molecules in transcriptome of Littorina littorea hemocytes

Transcriptome sequencing of the endangered land snail Karaftohelix adamsi from the Island Ulleung: De novo assembly, annotation, valuation of fitness genes and SSR markers

BACKGROUND

The Bradybaenidae snail Karaftohelix adamsi is endemic to Korea, with the species tracked from Island Ulleung in North Gyeongsang Province of South Korea. K. adamsi has been classified under the Endangered Wildlife Class II species of Korea and poses a severe risk of extinction following habitat disturbances. With no available information at the DNA (genome) or mRNA (transcriptome) level for the species, conservation by utilizing informed molecular resources seems difficult.

OBJECTIVE

In this study, we used the Illumina short-read sequencing and Trinity de novo assembly to draft the reference transcriptome of K. adamsi.

RESULTS

After assembly, 13,753 unigenes were obtained of which 10,511 were annotated to public databases (a maximum of 10,165 unigenes found homologs in PANM DB). A total of 6,351, 3,535, 358, and 3,407 unigenes were ascribed to the functional categories under KOG, GO, KEGG, and IPS, respectively. The transcripts such as the HSP 70, aquaporin, TLR, and MAPK, among others, were screened as putative functional resources for adaptation. DNA transposons were found to be thickly populated in comparison to retrotransposons in the assembled unigenes. Further, 2,164 SSRs were screened with the promiscuous presence of dinucleotide repeats such as AC/GT and AG/CT.

CONCLUSION

The transcriptome-guided discovery of molecular resources in K. adamsi will not only serve as a basis for functional genomics studies but also provide sustainable tools to be utilized for the protection of the species in the wild. Moreover, the development of polymorphic SSRs is valuable for the identification of species from newer habitats and cross-species genotyping.

Unveiling novel RNA viruses in trematodes parasitizing common periwinkle: Implications for host-parasite interactions

This study characterized novel RNA viruses, parasites of parasites, or hyperparasites identified during transcriptomic analyses of two trematode species, Cryptocotyle lingua and Himasthla elongata, infecting a sea snail, Littorina littorea. According to the viral genome structures and phylogenetic analysis, Cryptolin alternavirus (ClRNAV1), Cryptolin calicivirus (ClRNAV2) and Himastelon rhabdovirus 1 (HeRNAV1) were respectively classified within the families Alternaviridae, Caliciviridae and Rhabdoviridae. They replicate species-specifically in two studied phases of trematode live cycle: intramolluscan parthenogenetic rediae and free-swimming cercariae. ClRNAV1 showed significantly higher expression in C. lingua cercariae relative to rediae. HeRNAV1's similarity to rabies viruses raises questions about its potential effects on the nervous system of H. elongata. This 'trematode rabies' could enable the use of genetically modified viruses for developing new methods to control the spread and intensity of diseases caused by trematodes.

The transcriptomic analysis of Planorbarius corneus hemocytes (Gastropoda) naturally infected with Bilharziella polonica (Schistosomatidae)

The study of molluscan innate immunity is essential for understanding the evolution of the immune system. An advance in the knowledge of their immune system can be achieved by increasing the number of model species. Our study focuses on the immunity of Planorbarius corneus, a pulmonate snail widely distributed in Eurasia. These snails are intermediate hosts of many trematodes, including Bilharziella polonica (Schistosomatidae). In this paper we obtained and analyzed transcriptomes of hemocytes of uninfected snails Planorbarius corneus and snails naturally infected with Bilharziella polonica. The transcriptomes were found to contain transcripts encoding all major groups of immune factors previously described for other gastropods. Pathogen-recognition molecules were the most diverse group of immune factors. Comparison of the transcriptomes of the infected and the uninfected molluscs showed that the expression of some genes changed during infection. Our results extend the knowledge of immune responses of pulmonate snails to trematode invasion and promote P. corneus as a new model for the study of molluscan defence reactions.

Transcriptome analysis of the bivalve Placuna placenta mantle reveals potential biomineralization-related genes

The shells of window pane oyster Placuna placenta are very thin and exhibit excellent optical transparency and mechanical robustness. However, little is known about the biomineralization-related proteins of the shells of P. placenta. In this work, we report the comprehensive transcriptome of the mantle tissue of P. placenta for the first time. The unigenes of the mantle tissue of P. placenta were annotated by using the public databases such as nr, GO, KOG, KEGG, and Pfam. 24,343 unigenes were annotated according to Pfam database, accounting for 21.48% of the total unigenes. We find that half of the annotated unigenes of the mantle tissue of P. placenta are consistent to the annotated unigenes from pacific oyster Crassostrea gigas according to nr database. The unigene sequence analysis from the mantle tissue of P. placenta indicates that 465,392 potential single nucleotide polymorphisms (SNPs) and 62,103 potential indel markers were identified from 60,371 unigenes. 178 unigenes of the mantle tissue of P. placenta are found to be homologous to those reported proteins related to the biomineralization process of molluscan shells, while 18 of them are highly expressed unigenes in the mantle tissue. It is proposed that four unigenes with the highest expression levels in the mantle tissue are very often related to the biomineralization process, while another three unigenes are potentially related to the biomineralization process according to the Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) analysis. In summary, the transcriptome analysis of the mantle tissue of P. Placenta shows the potential biomineralization-related proteins and this work may shed light for the shell formation mechanism of bivalves.

Long-term exposure to environmental diclofenac concentrations impairs growth and induces molecular changes in Lymnaea stagnalis freshwater snails

As pharmaceutical substances are highly used in human and veterinary medicine and subsequently released in the environment, they represent emerging contaminants in the aquatic compartment. Diclofenac (DCF) is one of the most commonly detected pharmaceuticals in water and little research has been focused on its long-term effects on freshwater invertebrates. In this study, we assessed the chronic impacts of DCF on the freshwater gastropod Lymnaea stagnalis using life history, behavioral and molecular approaches. These organisms were exposed from the embryo to the adult stage to three environmentally relevant DCF concentrations (0.1, 2 and 10 μg/L). The results indicated that DCF impaired shell growth and feeding behavior at the juvenile stage, yet no impacts on hatching, locomotion and response to light stress were noted. The molecular findings (metabolomics and transcriptomic) suggested that DCF may disturb the immune system, energy metabolism, osmoregulation and redox balance. In addition, prostaglandin synthesis could potentially be inhibited by DCF exposure. The molecular findings revealed signs of reproduction impairment but this trend was not confirmed by the physiological tests. Combined omics tools provided complementary information and enabled us to gain further insight into DCF effects in freshwater organisms.

Nuclear genome of Bulinus truncatus, an intermediate host of the carcinogenic human blood fluke Schistosoma haematobium

Some snails act as intermediate hosts (vectors) for parasitic flatworms (flukes) that cause neglected tropical diseases, such as schistosomiases. Schistosoma haematobium is a blood fluke that causes urogenital schistosomiasis and induces bladder cancer and increased risk of HIV infection. Understanding the molecular biology of the snail and its relationship with the parasite could guide development of an intervention approach that interrupts transmission. Here, we define the genome for a key intermediate host of S. haematobium—called Bulinus truncatus—and explore protein groups inferred to play an integral role in the snail’s biology and its relationship with the schistosome parasite. Bu. truncatus shared many orthologous protein groups with Biomphalaria glabrata—the key snail vector for S. mansoni which causes hepatointestinal schistosomiasis in people. Conspicuous were expansions in signalling and membrane trafficking proteins, peptidases and their inhibitors as well as gene families linked to immune response regulation, such as a large repertoire of lectin-like molecules. This work provides a sound basis for further studies of snail-parasite interactions in the search for targets to block schistosomiasis transmission.

The snail Bulinus truncatus is an intermediate host of the carcinogenic human blood fluke Schistosoma haematobium. Here the authors report the genome of Bu. truncatus, explore protein groups inferred to play a role in its interaction with the schistosome parasite, and identify expansions in gene families linked to immune response regulation.

First Insights into the Repertoire of Secretory Lectins in Rotifers

Due to their high biodiversity and adaptation to a mutable and challenging environment, aquatic lophotrochozoan animals are regarded as a virtually unlimited source of bioactive molecules. Among these, lectins, i.e., proteins with remarkable carbohydrate-recognition properties involved in immunity, reproduction, self/nonself recognition and several other biological processes, are particularly attractive targets for biotechnological research. To date, lectin research in the Lophotrochozoa has been restricted to the most widespread phyla, which are the usual targets of comparative immunology studies, such as Mollusca and Annelida. Here we provide the first overview of the repertoire of the secretory lectin-like molecules encoded by the genomes of six target rotifer species: Brachionus calyciflorus, Brachionus plicatilis, Proales similis (class Monogononta), Adineta ricciae, Didymodactylos carnosus and Rotaria sordida (class Bdelloidea). Overall, while rotifer secretory lectins display a high molecular diversity and belong to nine different structural classes, their total number is significantly lower than for other groups of lophotrochozoans, with no evidence of lineage-specific expansion events. Considering the high evolutionary divergence between rotifers and the other major sister phyla, their widespread distribution in aquatic environments and the ease of their collection and rearing in laboratory conditions, these organisms may represent interesting targets for glycobiological studies, which may allow the identification of novel carbohydrate-binding proteins with peculiar biological properties.

Identification of peptidoglycan recognition proteins in hemocytes and kidney of common periwinkle Littorinalittorea

Peptidoglycan Recognition Proteins (PGRPs) are a diverse group of proteins involved in innate immunity. In particular, PGRPs have been shown to participate in immune pattern recognition in various mollusks. However, they have not been described in Caenogastropoda, a large molluscan group comprising sea, freshwater and land snails. In this study, four short PGRPs with molecular weights ranging from 21 to 34 kDa and their isoforms were identified and structurally characterized in the kidney and hemocytic transcriptomes of a caenogastropod mollusk Littorina littorea. All of them (LlPGRP1-4) are secretory, possess a signal peptide and a characteristic N-terminal N-acetylmuramoyl-l-alanine amidase (Ami) domain with conserved Zn²⁺ binding- and amidase catalytic sites. The shortest proteins, LlPGRP1 and LlPGRP2, have no additional conserved motifs on the N-terminus. In longer and most abundantly expressed LlPGRP3 and LlPGRP4 the Ami-domain is combined with an N-terminal SH3-domain and a cysteine-rich motif, respectively. Expression analysis showed that LlPGRPs of the common periwinkle were uninvolved in the immune response to infection with trematode Himasthla elongata though they might act in antibacterial defense.

Identification and diversity of fibrinogen-related protein (FREP) gene family in Haliotis discus hannai, H. rufescens, and H. laevigata and their responses to Vibrio parahemolyticus infection

Fibrinogen-related proteins (FREPs) are distributed universally in vertebrates and invertebrates. These proteins contain fibrinogen-like (FBG) domains in their C-terminal region and involve in immune responses and other aspects of physiology in invertebrates. In this study, 54 proteins that contain FBG domains or a fibrinogen_c domain were identified in Haliotis discus hannai. Comparatively, 88 and 63 FREPs were identified from the genomes of H. rufescens and H. laevigata. Most FREPs of abalones had a conserved motif containing a bound calcium ion site and a second conserved motif containing a polymerization pocket site. By sequence analysis, 394 SNPs and 11 Indels were identified in 20 FREP genes of the whole genome of H. discus hannai; 992 SNPs and 42 Indels were found in 64 FREPs of H. rufescens, and 192 SNPs and 12 Indels were found in 21 FREPs of H. laevigata. Among these SNPs, 92 missense mutation sites were identified in 26 FREP genes of H. rufescens, and 12 were identified in 8 FREP genes of H. laevigata. Due to the poor genomic integrity, annotations of the SNPs or Indels in H. discus hannai did not yield missense mutant sites. FREP genes with polymorphisms were ubiquitously expressed in all the tested tissues; however, the expression is lowest in the hemolymph. In response to Vibrio parahemolyticus infection, expression of FREP genes was significantly upregulated at different exposure times in gills, hepatopancreas, and hemolymph in H. discus hannai. Overall, this study documented the FREP genes of abalones and shed light on the role of FREPs in the innate immune system of these aquaculture species for the prevention and control of diseases.

Species-Specific Proteins in the Oviducts of Snail Sibling Species: Proteotranscriptomic Study of Littorina fabalis and L. obtusata

Genus Littorina subgenus Neritrema (Mollusca, Caenogastropoda) includes the "obtusata" group of closely related species (Littorina obtusata and L. fabalis). The anatomy of the adult reproductive system (pallial oviduct) is the only reliable feature used for species identification in females of these species. Reproductive system anatomy and reproduction-associated proteins often diverge between sibling species. Despite being of high evolutionary interest, the molecular basis of this divergence remains poorly understood. We performed proteotranscriptomic comparison of oviducts of L. obtusata and L. fabalis by RNA-seq on Illumina HiSeq 2500 and two-dimensional protein electrophoresis (2D DIGE) with MS/MS identification of the species-specific proteins. The interspecies differences in the oviduct were associated with (1) metabolic proteins reflecting overall physiological differences between L. obtusata and L. fabalis, (2) receptor proteins, and (3) transcripts related to transposable elements (TEs). Various receptors identified may recognize a wide variety of ligands from pathogen-associated molecular patterns to specific carbohydrates on the sperm surface. Therefore, these may participate in immune defense as well as in sperm storage and regulation. Species-specificity of multiple TE sequences (coding for reverse transcriptase and ribonuclease H) may indicate the important role of these genomic elements in the Littorina species divergence, which has not been reported previously.

Examining adaptive evolution of immune activity: opportunities provided by gastropods in the age of 'omics'

Parasites threaten all free-living organisms, including molluscs. Understanding the evolution of immune defence traits in natural host populations is crucial for predicting their long-term performance under continuous infection risk. Adaptive trait evolution requires that traits are subject to selection (i.e. contribute to organismal fitness) and that they are heritable. Despite broad interest in the evolutionary ecology of immune activity in animals, the understanding of selection on and evolutionary potential of immune defence traits is far from comprehensive. For instance, empirical observations are only rarely in line with theoretical predictions of immune activity being subject to stabilizing selection. This discrepancy may be because ecoimmunological studies can typically cover only a fraction of the complexity of an animal immune system. Similarly, molecular immunology/immunogenetics studies provide a mechanistic understanding of immunity, but neglect variation that arises from natural genetic differences among individuals and from environmental conditions. Here, we review the current literature on natural selection on and evolutionary potential of immune traits in animals, signal how merging ecological immunology and genomics will strengthen evolutionary ecological research on immunity, and indicate research opportunities for molluscan gastropods for which well-established ecological understanding and/or 'immune-omics' resources are already available. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

Transcriptome profiling of Lymnaea stagnalis (Gastropoda) for ecoimmunological research

BACKGROUND

Host immune function can contribute to numerous ecological/evolutionary processes. Ecoimmunological studies, however, typically use one/few phenotypic immune assays and thus do not consider the complexity of the immune system. Therefore, "omics" resources that allow quantifying immune activity across multiple pathways are needed for ecoimmunological models. We applied short-read based RNAseq (Illumina NextSeq 500, PE-81) to characterise transcriptome profiles of Lymnaea stagnalis (Gastropoda), a multipurpose model snail species. We used a genetically diverse snail stock and exposed individuals to immune elicitors (injury, bacterial/trematode pathogens) and changes in environmental conditions that can alter immune activity (temperature, food availability).

RESULTS

Immune defence factors identified in the de novo assembly covered elements broadly described in other gastropods. For instance, pathogen-recognition receptors (PRR) and lectins activate Toll-like receptor (TLR) pathway and cytokines that regulate cellular and humoral defences. Surprisingly, only modest diversity of antimicrobial peptides and fibrinogen related proteins were detected when compared with other taxa. Additionally, multiple defence factors that may contribute to the phenotypic immune assays used to quantify antibacterial activity and phenoloxidase (PO)/melanisation-type reaction in this species were found. Experimental treatments revealed factors from non-self recognition (lectins) and signalling (TLR pathway, cytokines) to effectors (e.g., antibacterial proteins, PO enzymes) whose transcription depended on immune stimuli and environmental conditions, as well as components of snail physiology/metabolism that may drive these effects. Interestingly, the transcription of many factors (e.g., PRR, lectins, cytokines, PO enzymes, antibacterial proteins) showed high among-individual variation.

CONCLUSIONS

Our results indicate several uniform aspects of gastropod immunity, but also apparent differences between L. stagnalis and some previously examined taxa. Interestingly, in addition to immune defence factors that responded to immune elicitors and changes in environmental conditions, many factors showed high among-individual variation across experimental snails. We propose that such factors are highly important to be included in future ecoimmunological studies because they may be the key determinants of differences in parasite resistance among individuals both within and between natural snail populations.

Quadruple domain-containing galectin from marine invertebrate disk abalone (Haliotis discus discus): Molecular perspectives in early development, immune expression, and potent antiviral responses

Galectins are well-known β-galactoside-binding proteins, which play vital roles in innate immune responses of both vertebrates and invertebrates. However, knowledge regarding invertebrate galectins is still in its infancy. With the intention of filling the knowledge gap, here we identified a quadruple domain-containing galectin from marine invertebrate disk abalone, Haliotis discus discus (AbGalec), and characterized it. AbGalec consisted of four distinct carbohydrate-recognition domains (CRDs) and lacked a signal peptide. Expression analysis revealed AbGalec to be ubiquitously expressed in all the examined early embryonic stages of abalone, with highest expression in the 16-cell stage, suggesting the importance of AbGalec in early developmental processes. Tissue distribution analysis revealed the highest expression of AbGalec in abalone mantle, followed by that in gills and hemocytes. Immune challenge experiments revealed significant upregulation of AbGalec at 24 h and 48 h post injection (p.i.) with bacterial and viral components. These results suggested the possible involvement of AbGalec in host defense mechanisms. Polyinosinic: polycytidylic acid (Poly I:C) and viral hemorrhagic septicemia virus (VHSV) injections were capable of inducing AbGalec transcript expression more prominently than bacterial stimulants, thus providing evidence for its role in viral infections. We determined the virus-neutralizing ability of a quadruple domain-containing galectin for the first time, by analyzing the downregulation of VHSV transcripts during the overexpression of AbGalec. Significant downregulation of VHSV transcripts was observed after 24 h and 48 h of post infection. Collectively, our findings reveal the potent antiviral responses of molluscan quadruple domain-containing galectin, AbGalec, along with its involvement in innate immunity.

In-Depth In Silico Search for Cuttlefish (Sepia officinalis) Antimicrobial Peptides Following Bacterial Challenge of Haemocytes

Cuttlefish (Sepia officinalis) haemocytes are potential sources of antimicrobial peptides (AMPs). To study the immune response to Vibrio splendidus and identify new AMPs, an original approach was developed based on a differential transcriptomic study and an in-depth in silico analysis using multiple tools. Two de novo transcriptomes were retrieved from cuttlefish haemocytes following challenge by V. splendidus or not. A first analysis of the annotated transcripts revealed the presence of Toll/NF-κB pathway members, including newly identified factors such as So-TLR-h, So-IKK-h and So-Rel/NF-κB-h. Out of the eight Toll/NF-κB pathway members, seven were found up-regulated following V. splendidus challenge. Besides, immune factors involved in the immune response were also identified and up-regulated. However, no AMP was identified based on annotation or conserved pattern searches. We therefore performed an in-depth in silico analysis of unannotated transcripts based on differential expression and sequence characteristics, using several tools available like PepTraq, a homemade software program. Finally, five AMP candidates were synthesized. Among them, NF19, AV19 and GK28 displayed antibacterial activity against Gram-negative bacteria. Each peptide had a different spectrum of activity, notably against Vibrio species. GK28—the most active peptide—was not haemolytic, whereas NF19 and AV19 were haemolytic at concentrations between 50 and 100 µM, 5 to 10 times higher than their minimum inhibitory concentration.

Multi-omics investigations within the Phylum Mollusca, Class Gastropoda: from ecological application to breakthrough phylogenomic studies

Gastropods are the largest and most diverse class of mollusc and include species that are well studied within the areas of taxonomy, aquaculture, biomineralization, ecology, microbiome and health. Gastropod research has been expanding since the mid-2000s, largely due to large-scale data integration from next-generation sequencing and mass spectrometry in which transcripts, proteins and metabolites can be readily explored systematically. Correspondingly, the huge data added a great deal of complexity for data organization, visualization and interpretation. Here, we reviewed the recent advances involving gastropod omics ('gastropodomics') research from hundreds of publications and online genomics databases. By summarizing the current publicly available data, we present an insight for the design of useful data integrating tools and strategies for comparative omics studies in the future. Additionally, we discuss the future of omics applications in aquaculture, natural pharmaceutical biodiscovery and pest management, as well as to monitor the impact of environmental stressors.

Differences in proteomic profile between two haemocyte types, granulocytes and hyalinocytes, of the flat oyster Ostrea edulis

Haemocytes play a dominant role in shellfish immunity, being considered the main defence effector cells in molluscs. These cells are known to be responsible for many functions, including chemotaxis, cellular recognition, attachment, aggregation, shell repair and nutrient transport and digestion. There are two basic cell types of bivalve haemocytes morphologically distinguishable, hyalinocytes and granulocytes; however, functional differences and specific abilities are poorly understood: granulocytes are believed to be more efficient in killing microorganisms, while hyalinocytes are thought to be more specialised in clotting and wound healing. A proteomic approach was implemented to find qualitative differences in the protein profile between granulocytes and hyalinocytes of the European flat oyster, Ostrea edulis, as a way to evaluate functional differences. Oyster haemolymph cells were differentially separated by Percoll® density gradient centrifugation. Granulocyte and hyalinocyte proteins were separated by 2D-PAGE and their protein profiles were analysed and compared with PD Quest software; the protein spots exclusive for each haemocyte type were excised from gels and analysed by MALDI-TOF/TOF with a combination of mass spectrometry (MS) and MS/MS for sequencing and protein identification. A total of 34 proteins were identified, 20 unique to granulocytes and 14 to hyalinocytes. The results suggested differences between the haemocyte types in signal transduction, apoptosis, oxidation reduction processes, cytoskeleton, phagocytosis and pathogen recognition. These results contribute to identify differential roles of each haemocyte type and to better understand the oyster immunity mechanisms, which should help to fight oyster diseases.

Physiology and immunity of the invasive giant African snail, Achatina (Lissachatina) fulica, intermediate host of Angiostrongylus cantonensis

As one of the most successful invasive land snail species, Achatina (Lissachatina) fulica Bowdich, 1822 has achieved wide global distribution, particularly in (sub)tropical regions, with further dispersal likely due to climate change. This species of giant African snails (up to 17 cm shell length) is a pest that has extensive negative impact on agriculture and can serve as vector for several parasites, including Angiostrongylus cantonensis, a nematode parasite that causes (human) eosinophilic meningitis, an emergent disease. Investigation showed that A. cantonensis infection negatively impacts the metabolism of A. fulica by depleting polysaccharide stores of the intermediate host, compromising the energy balance of the snail. A review of the literature indicates that A. fulica possesses potent innate type immune defenses to counter infection, including phagocytic hemocytes capable of deploying reactive oxygen species and lectins for non-self recognition, a serine protease-dependent coagulation response (not observed in other taxa of gastropods), as well as antimicrobial proteins including achacin, an antimicrobial protein. A recent chromosome level genome assembly will facilitate progressively detailed characterization of these immune features of A. fulica. We strongly encourage further immunological studies of A. fulica, ranging from organismal level to molecular biology to gain better understanding of the A. fulica internal defense response to nematode pathogens like A. cantonensis and the contribution of immune function to the invasiveness of (snail) species. Characterization of immunity of A. fulica, representing the understudied Stylommatophora (panpulmonate landsnails) will also broaden the comparative immunology of Gastropoda.

Transcriptomic resources for evolutionary studies in flat periwinkles and related species

The flat periwinkles, Littorina fabalis and L. obtusata, comprise two sister gastropod species that have an enormous potential to elucidate the mechanisms involved in ecological speciation in the marine realm. However, the molecular resources currently available for these species are still scarce. In order to circumvent this limitation, we used RNA-seq data to characterize the transcriptome of four individuals from each species sampled in different locations across the Iberian Peninsula. Four de novo transcriptome assemblies were generated, as well as a pseudo-reference using the L. saxatilis reference transcriptome as backbone. After transcripts' annotation, variant calling resulted in the identification of 19,072 to 45,340 putatively species-diagnostic SNPs. The discriminatory power of a subset of these SNPs was validated by implementing an independent genotyping assay to characterize reference populations, resulting in an accurate classification of individuals into each species and in the identification of hybrids between the two. These data comprise valuable genomic resources for a wide range of evolutionary and conservation studies in flat periwinkles and related taxa.

Biomphalaria glabrata immunity: Post-genome advances

The freshwater snail, Biomphalaria glabrata, is an important intermediate host in the life cycle for the human parasite Schistosoma mansoni, the causative agent of schistosomiasis. Current treatment and prevention strategies have not led to a significant decrease in disease transmission. However, the genome of B. glabrata was recently sequenced to provide additional resources to further our understanding of snail biology. This review presents an overview of recently published, post-genome studies related to the topic of snail immunity. Many of these reports expand on findings originated from the genome characterization. These novel studies include a complementary gene linkage map, analysis of the genome of the B. glabrata embryonic (Bge) cell line, as well as transcriptomic and proteomic studies looking at snail-parasite interactions and innate immune memory responses towards schistosomes. Also included are biochemical investigations on snail pheromones, neuropeptides, and attractants, as well as studies investigating the frontiers of molluscan epigenetics and cell signaling were also included. Findings support the current hypotheses on snail-parasite strain compatibility, and that snail host resistance to schistosome infection is dependent not only on genetics and expression, but on the ability to form multimeric molecular complexes in a timely and tissue-specific manner. The relevance of cell immunity is reinforced, while the importance of humoral factors, especially for secondary infections, is supported. Overall, these studies reflect an improved understanding on the diversity, specificity, and complexity of molluscan immune systems.

Molecular characterization and expression analysis of fibrinogen related protein (FREP) genes of Manila clam (Ruditapes philippinarum) after lipopolysaccharides challenge

Ruditapes philippinarum has high economic value and is distributed all over the world. Fibrinogen associated protein (FREP) is a type of pattern recognition receptor, participates in the innate immune response to eliminate pathogens after the invasion of pathogenic microorganisms. In this study, three FREP genes (FREP-1, FREP-2, and FREP-3) were identified and characterized from R. philippinarum. The protein sequence of FREPs were highly conserved with those homologous in vertebrates, and FBG domain possessed significantly high structural conservation in polypeptide binding site and Ca²⁺ binding site. The tissues expression analysis of FREPs in three shell color strains and two population of R. philippinarum were examined, with the highest expression level in gill and hepatopancreas. Besides, FREP genes were demonstrated to be induced by lipopolysaccharides injection, the significantly changes were observed after LPS injected. Our results suggest the involvement of FREPs in response to LPS injection, and it might exert a significant function on the innate immune defense of the Manila clam.

Immune response of a caenogastropod host: A case study of Littorina littorea and its digenean parasites

The caenogastropod mollusk Littorina littorea is a promising experimental model for comparative studies on host/parasite immune conflict. Several different digenean parasites use L. littorea as the first intermediate host, overcoming snail immune reactions by a wide range of tactics that are radically different among different digenean species and at different developmental parasite stages. The immune system of L. littorea is rather effective against digenean Himasthla elongata invasion, and even successfully established parasite induces a chronic host immune reaction, present at a low but stable level, that may be involved in the selection of derived parasitic clones in long lived self-sustaining infrapopulations (SSI) of rediae. An anti-digenean response in L. littorea is not systemic (non-generalized) yet tissue specific, mostly reliant on cellular rather than humoral reactions. The repertoire of immune pattern-recognizing receptors in the common periwinkle comprises diverse secreted and membrane-attached lectin molecules, as the main drivers of snail immune discrimination of digenean parasites. Comparative studies suggest that the characteristic vulnerability to digenean parasitism of L. littorea, and gastropods in general, is in part due the overall organization of immunity relative to other classes of molluscs, e.g. the immune strategy of bivalves seems to rely on less specific cellular reactions and a more generalized systemic humoral immunity. This difference may arise from the molecular features of the selective retention of their taxon-specific complement-like molecular complexes, which diverged in common ancestors of Bivalvia and Gastropoda.

Metal stress-related gene expression patterns in two marine invertebrates, Hediste diversicolor (Annelida, Polychaeta) and Littorina littorea (Mollusca, Gastropoda), at a former mining site

Abandoned mines often contaminate sediments with dissolved heavy metals and are known to impact many taxa. Physiological responses needed to avoid oxidative stress from metal toxicity include the upregulation of detoxification and metal-binding proteins such as glutathione-s-transferases and metallothioneins, which have been studied in diverse species. Fewer studies, however, have focused on gene expression changes to better understand these molecular mechanisms, especially across multiple species at a single contaminated site. To this end, the purpose of this study was to characterize metal stress-related gene expression in two species from different phyla, Hediste diversicolor (Annelida, Polychaeta) and Littorina littorea (Mollusca, Gastropoda), at a former mine site (Callahan Mine, Maine, USA). Both species and sediments were collected from a mine-affected tidal estuary (Goose Pond) and a nearby reference site. Elevated sediment metal levels were confirmed at Goose Pond. H. diversicolor individuals weighed significantly less at Goose Pond, while L. littorea weighed similarly at both sites. Transcript levels were stable in H. diversicolor but weakly upregulated in L. littorea, which likely reflect the importance of other physiological strategies for metal sequestration, or variable metal exposure at the individual level, respectively. In addition, patterns in glutathione-s-transferase expression differed across isoforms in H. diversicolor, while L. littorea exhibited divergent expression patterns in foot muscle and hepatopancreas. Overall, these results reinforce that diverse species likely undergo different physiological responses to metal toxicity, and more research is needed to investigate these mechanisms.

Identification and characterisation of anti - Pseudomonas aeruginosa proteins in mucus of the brown garden snail, Cornu aspersum

Background: Novel antimicrobial treatments are urgently needed. Previous work has shown that the mucus of the brown garden snail (Cornu aspersum) has antimicrobial properties, in particular against type culture collection strains of Pseudomonas aeruginosa. We hypothesised that it would also be effective against clinical isolates of the bacterium and that investigation of fractions of the mucus would identify one or more proteins with anti-pseudomonal properties, which could be further characterised. Materials and methods: Mucus was extracted from snails collected from the wild. Antimicrobial activity against laboratory and clinical isolates of Ps. aeruginosa was determined in disc diffusion assays. Mucus was purified using size exclusion chromatography and fractions containing anti-pseudomonal activity identified. Mass spectroscopy and high performance liquid chromatography analysis of these fractions yielded partial peptide sequences. These were used to interrogate an RNA transcriptome generated from whole snails. Results: Mucus from C. aspersum inhibited growth of type collection strains and clinical isolates of Ps. aeruginosa. Four novel C. aspersum proteins were identified; at least three are likely to have antimicrobial properties. The most interesting is a 37.4 kDa protein whilst smaller proteins, one 17.5 kDa and one 18.6 kDa also appear to have activity against Ps. aeruginosa. Conclusions: The study has identified novel proteins with antimicrobial properties which could be used to develop treatments for use in human medicine.

Divergent immune roles of two fucolectin isoforms in Apostichopus japonicus

The F-type lectin (fucolectin) family represents a new group with innate immunity. In this study, two fucolectin isoforms (designated as AjFTL-1 and AjFTL-2) were identified in sea cucumber (Apostichopus japonicus) through rapid amplification of cDNA ends. Full-length cDNAs of AjFTL-1 and AjFTL-2 measured 2134 and 1286 bp, encoding two secreted proteins comprising 317 and 181 amino acid residues, respectively. The signal peptide, l-fucose binding motif ("HX(26)RXDX(4)R/K") and cation binding sequence motif ("h2DGx") were conserved in AjFTL-1 and AjFTL-2. However, AjFTL-1 contains an additional complement control protein domain. Multiple sequence alignments supported that AjFTL-1 and AjFTL-2 are new members of the F-type lectin family. Tissues distribution analysis indicated that both AjFTL-1 and AjFTL-2 were widely expressed in all tested tissues, featuring differential expression patterns. Vibrio splendidus infection in vivo can significantly upregulate the mRNA transcripts of the two genes, with a larger magnitude observed in AjFTL-1. By contrast, lipopolysaccharide stimulation in vitro can markedly induce the expression level of AjFTL-2 but not that of AjFTL-1. Silencing AjFTL-2 by siRNA can suppress the AjNOS transcript, whereas injection of the recombinant protein of AjFTL-2 can significantly induce AjNOS expression. By contrast, the loss- and gain-of-function of AjFTL-1 caused no effect on the expression of AjNOS. Our present study provides evidence supporting that AjFTL-1 and AjFTL-2 play diverse roles in the innate immune defense of sea cucumbers toward bacterial infection.

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 immunological study of the snail Physella acuta (Hygrophila, Pulmonata) reveals shared and unique aspects of gastropod immunobiology

The freshwater snail Physella acuta was selected to expand the perspective of comparative snail immunology. Analysis of Physella acuta, belonging to the Physidae, taxonomic sister family to Planorbidae, affords family-level comparison of immune features characterized from Biomphalaria glabrata, the model snail often used to interpret general gastropod immunity. To capture constitutive and induced immune sequences, transcriptomes of an individual Physella acuta snail, 12 h post injection with bacteria (Gram -/+) and one sham-exposed snail were recorded with 454 pyrosequencing. Assembly yielded a combined reference transcriptome containing 24,288 transcripts. Additionally, genomic Illumina reads were obtained (∼15-fold coverage). Recovery of transcripts for two macin-like antimicrobial peptides (AMPs), 12 aplysianins, four LBP/BPIs and three physalysins indicated that Physella acuta shares a similar organization of antimicrobial defenses with Biomphalaria glabrata, contrasting a modest AMP arsenal with a diverse set of antimicrobial proteins. The lack of predicted transmembrane domains in all seven Physella acuta PGRP transcripts supports the notion that gastropods do not employ cell-bound PGRP receptors, different from ecdysozoan invertebrates yet similar to mammals (vertebrate deuterostomes). The well-documented sequence diversification by Biomphalaria glabrata FREPs (immune lectins comprising immunoglobulin superfamily domains and fibrinogen domains), resulting from somatic mutations of a large FREP gene family is hypothesized to be unique to Planorbidae; Physella acuta revealed just two bonafide FREP genes and these were not diversified. Furthermore, the flatworm parasite Echinostoma paraensei, confirmed here to infect both snail species, did not evoke from Physella acuta the abundant expression of FREP proteins at 2, 4 and 8 days post exposure that was previously observed from Biomphalaria glabrata. The Physella acuta reference transcriptome also revealed 24 unique transcripts encoding proteins consisting of a single fibrinogen-related domain (FReDs), with a short N-terminal sequence encoding either a signal peptide, transmembrane domain or no predicted features. The Physella acuta FReDs are candidate immune genes based on implication of similar sequences in immunity of bivalve molluscs. Overall, comparative analysis of snails of sister families elucidated the potential for taxon-specific immune features and investigation of strategically selected species will provide a more comprehensive view of gastropod immunity.

F-Type Lectins: A Highly Diversified Family of Fucose-Binding Proteins with a Unique Sequence Motif and Structural Fold, Involved in Self/Non-Self-Recognition

The F-type lectin (FTL) family is one of the most recent to be identified and structurally characterized. Members of the FTL family are characterized by a fucose recognition domain [F-type lectin domain (FTLD)] that displays a novel jellyroll fold ("F-type" fold) and unique carbohydrate- and calcium-binding sequence motifs. This novel lectin family comprises widely distributed proteins exhibiting single, double, or greater multiples of the FTLD, either tandemly arrayed or combined with other structurally and functionally distinct domains, yielding lectin subunits of pleiotropic properties even within a single species. Furthermore, the extraordinary variability of FTL sequences (isoforms) that are expressed in a single individual has revealed genetic mechanisms of diversification in ligand recognition that are unique to FTLs. Functions of FTLs in self/non-self-recognition include innate immunity, fertilization, microbial adhesion, and pathogenesis, among others. In addition, although the F-type fold is distinctive for FTLs, a structure-based search revealed apparently unrelated proteins with minor sequence similarity to FTLs that displayed the FTLD fold. In general, the phylogenetic analysis of FTLD sequences from viruses to mammals reveals clades that are consistent with the currently accepted taxonomy of extant species. However, the surprisingly discontinuous distribution of FTLDs within each taxonomic category suggests not only an extensive structural/functional diversification of the FTLs along evolutionary lineages but also that this intriguing lectin family has been subject to frequent gene duplication, secondary loss, lateral transfer, and functional co-option.

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.

Proteomic analysis of Biomphalaria glabrata plasma proteins with binding affinity to those expressed by early developing larval Schistosoma mansoni

Interactions between early developing Schistosoma mansoni larval stages and the hemolymph of its snail intermediate host represent the first molecular encounter with the snail’s immune system. To gain a more comprehensive understanding of this early parasite-host interaction, biotinylated sporocyst tegumental membrane (Mem) proteins and larval transformation proteins (LTP) were affixed to streptavidin-agarose beads and used as affinity matrices to enrich for larval-reactive plasma proteins from susceptible (NMRI) and resistant (BS-90) strains of the snail Biomphalaria glabrata. Nano-LC/MS-MS proteomic analyses of isolated plasma proteins revealed a diverse array of 94 immune-and nonimmune-related plasma proteins. Included among the immune-related subset were pattern recognition receptors (lectins, LPS-binding protein, thioester-containing proteins-TEPs), stress proteins (HSP60 and 70), adhesion proteins (dermatopontins), metalloproteases (A Disintegrin And Metalloproteinase (ADAM), ADAM-related Zn proteinases), cytotoxins (biomphalysin) and a Ca²⁺-binding protein (neo-calmodulin). Variable immunoglobulin and lectin domain (VIgL) gene family members, including fibrinogen-related proteins (FREPs), galectin-related proteins (GREPs) and C-type lectin-related proteins (CREPs), were the most prevalent of larval-reactive immune lectins present in plasma. FREPs were highly represented, although only a subset of FREP subfamilies (FREP 2, 3 and 12) were identified, suggesting potential selectivity in the repertoire of plasma lectins recognizing larval glycoconjugates. Other larval-binding FREP-like and CREP-like proteins possessing a C-terminal fibrinogen-related domain (FReD) or C-type lectin binding domain, respectively, and an Ig-fold domain also were identified as predicted proteins from the B. glabrata genome, although incomplete sequence data precluded their placement into specific FREP/CREP subfamilies. Similarly, a group of FReD-containing proteins (angiopoeitin-4, ficolin-2) that lacked N-terminal Ig-fold(s) were identified as a distinct group of FREP-like proteins, separate from the VIgL lectin family. Finally, differential appearance of GREPs in BS-90 plasma eluates, and others proteins exclusively found in eluates of the NMRI strain, suggested snail strain differences in the expression of select larval-reactive immune proteins. This hypothesis was supported by the finding that differential gene expression of the GREP in BS-90 and ADAM in NMRI snail strains generally correlated with their patterns of protein expression. In summary, this study is the first to provide a global comparative proteomic analysis of constitutively expressed plasma proteins from susceptible and resistant B. glabrata strains capable of binding early-expressed larval S. mansoni proteins. Identified proteins, especially those exhibiting differential expression, may play a role in determining immune compatibility in this snail host-parasite system. A complete listing of raw peptide data are available via ProteomeXchange using identifier PXD004942.

Transmission of the human blood fluke Schistosoma mansoni critically depends on the successful establishment of infections within species of its snail intermediate host, Biomphalaria. One of the most important barriers to infection is the host’s innate immune system, comprised of plasma proteins and immunocytes (hemocytes) circulating in the hemolymph. Although expression of plasma lectin genes appears to be associated with larval resistance in B. glabrata, few studies have attempted an in depth analysis of gene-encoded lectins, and other immune proteins, that are capable of directly binding schistosome larvae. Using affinity matrices linked to schistosome proteins expressed during early larval development, we identified and compared the parasite-reactive plasma proteins from the susceptible NMRI and resistant BS-90 strains of B. glabrata. Proteomic analyses of isolated plasma proteins revealed a diversity immune-related proteins including lectins, pathogen recognition receptors, cytotoxins, adhesion proteins, metalloproteinases, and Ca²⁺-binding proteins. Of the lectins, the variable immunoglobulin and lectin domain (VIgL) gene family of proteins comprised of fibrinogen-related proteins (FREPs), galectin-related proteins (GREPs) and C-type lectin-related proteins (CREPs), were highly represented, and consistent with their role in host immunity. Two proteins (GREP and a Zn-metalloproteinase) exhibited snail strain-associated protein and gene expression patterns suggesting their involvement in innate immune responses to larval infection. This comparative proteomic analysis of larval S. mansoni-reactive plasma proteins from susceptible and resistant B. glabrata strains represents the first of its kind and provides valuable insights into possible pathogen recognition receptors and other immune factors regulating parasite-host compatibility in this model system.

Differential Protein Expression in the Hemolymph of Bithynia siamensis goniomphalos Infected with Opisthorchis viverrini

Bithynia siamensis goniomphalos is a freshwater snail that serves as the first intermediate host of the human liver fluke Opisthorchis viverrini. This parasite is a major public health problem in different countries throughout the Greater Mekong sub-region (Thailand, southern Vietnam, Lao PDR and Cambodia). Chronic O. viverrini infection also results in a gradual increase of fibrotic tissues in the biliary tract that are associated with hepatobiliary diseases and contribute to cholangiocarcinoma (a fatal type of bile duct cancer). Infectivity of the parasite in the snail host is strongly correlated with destruction of helminths by the snail's innate immune system, composed of cellular (hemocyte) and humoral (plasma) defense factors. To better understand this important host-parasite interface we applied sequential window acquisition of all theoretical spectra mass spectrometry (SWATH-MS) to identify and quantify the proteins from the hemolymph of B. siamensis goniomphalos experimentally infected with O. viverrini and compare them to non-infected snails (control group). A total of 362 and 242 proteins were identified in the hemocytes and plasma, respectively. Of these, 145 and 117 proteins exhibited significant differences in expression upon fluke infection in hemocytes and plasma, respectively. Among the proteins with significantly different expression patterns, we found proteins related to immune response (up-regulated in both hemocyte and plasma of infected snails) and proteins belonging to the structural and motor group (mostly down-regulated in hemocytes but up-regulated in plasma of infected snails). The proteins identified and quantified in this work will provide important information for the understanding of the factors involved in snail defense against O. viverrini and might facilitate the development of new strategies to control O. viverrini infection in endemic areas.

Membrane Attack Complex/Perforin domain-containing proteins in a dual-species transcriptome of caenogastropoda Littorina littorea and its trematode parasite Himasthla elongata

The MACPF domain-containing proteins potentially able to build a transmembrane pore structure are found in the transcriptome of a common periwinkle kidney loaded with trematode rediae. Two homologs of mammalian Macrophage expressed gene 1 (Mpeg1), LlMpeg1-1 and LIMpeg1-2, share similar domain structure with the only difference such as LIMpeg1-2 transcript lacks of a C-terminal transmembrane helix. Expression of membrane-anchored protein LlMpeg1-1 is similar in kidneys of naturally infected with trematode Himasthla elongata and uninfected snails. The expression of the second soluble LIMpeg1-2 protein is 4-fold upregulated under infection. The third MACPF protein found in Littorina littorea kidney is homologous with Perivitellin - 2 67 kDa subunit named LlPV2-67 and the expression of the transcript is 3-fold upregulated in the kidney of infected snails. The last two molecules are candidate effectors that may participate in the immune response of common periwinkles to trematode infestation. A single parasite-expressed MACPF-like protein was recorded from the transcriptome of Himasthla elongata.

Sequencing, De Novo Assembly, and Annotation of the Transcriptome of the Endangered Freshwater Pearl Bivalve, Cristaria plicata, Provides Novel Insights into Functional Genes and Marker Discovery

BACKGROUND

The freshwater mussel Cristaria plicata (Bivalvia: Eulamellibranchia: Unionidae), is an economically important species in molluscan aquaculture due to its use in pearl farming. The species have been listed as endangered in South Korea due to the loss of natural habitats caused by anthropogenic activities. The decreasing population and a lack of genomic information on the species is concerning for environmentalists and conservationists. In this study, we conducted a de novo transcriptome sequencing and annotation analysis of C. plicata using Illumina HiSeq 2500 next-generation sequencing (NGS) technology, the Trinity assembler, and bioinformatics databases to prepare a sustainable resource for the identification of candidate genes involved in immunity, defense, and reproduction.

RESULTS

The C. plicata transcriptome analysis included a total of 286,152,584 raw reads and 281,322,837 clean reads. The de novo assembly identified a total of 453,931 contigs and 374,794 non-redundant unigenes with average lengths of 731.2 and 737.1 bp, respectively. Furthermore, 100% coverage of C. plicata mitochondrial genes within two unigenes supported the quality of the assembler. In total, 84,274 unigenes showed homology to entries in at least one database, and 23,246 unigenes were allocated to one or more Gene Ontology (GO) terms. The most prominent GO biological process, cellular component, and molecular function categories (level 2) were cellular process, membrane, and binding, respectively. A total of 4,776 unigenes were mapped to 123 biological pathways in the KEGG database. Based on the GO terms and KEGG annotation, the unigenes were suggested to be involved in immunity, stress responses, sex-determination, and reproduction. A total of 17,251 cDNA simple sequence repeats (cSSRs) were identified from 61,141 unigenes (size of >1 kb) with the most abundant being dinucleotide repeats.

CONCLUSIONS

This dataset represents the first transcriptome analysis of the endangered mollusc, C. plicata. The transcriptome provides a comprehensive sequence resource for the conservation of genetic information in this species and enrichment of the genetic database. The development of molecular markers will assist in the genetic improvement of C. plicata.

The role of fibrinogen-related proteins in the gastropod immune response

Fibrinogen-related proteins or FREPs constitute a large family of molecules, defined by the presence of a fibrinogen-related domain (FReD). These molecules are found in all animals and are diverse in both form and function. Here, we review the current understanding of gastropod FREPs, which are characterized by the presence of a fibrinogen domain connected to one or two immunoglobulin superfamily domains by way of a short interceding region. We present a historical perspective on the discovery of FREPs in gastropods followed by a summary of advances made in the nearly two decades of research focused on the characterization of FREPs in Biomphalaria glabrata (BgFREPs). Topics covered include BgFREP genomic architecture, predicted structure and known functions, structural comparisons between BgFREPs, and evidence of somatic diversification. Also examined are the expression patterns of BgFREPs during snail development and immunological challenges. Recent functional characterization of the role BgFREPs play in the defence response against digenean trematodes is also presented, as well as new data investigating the nucleotide-level genomic conservation of FREPs among Pulmonate gastropods. Finally, we identify areas in need of further research. These include confirming and identifying the specific binding targets of BgFREPs and elucidating how they later engage snail haemocytes to elicit an immunological response, precise mechanisms and importance of BgFREP diversification, characterizing the tissue expression patterns of BgFREPs, as well as addressing whether gastropod FREPs retain immunological importance in alternative snail-trematode associations or more broadly in snail-pathogen interactions.

Structural, functional, and evolutionary aspects of galectins in aquatic mollusks: From a sweet tooth to the Trojan horse

Galectins constitute a conserved and widely distributed lectin family characterized by their binding affinity for β-galactosides and a unique binding site sequence motif in the carbohydrate recognition domain (CRD). In spite of their structural conservation, galectins display a remarkable functional diversity, by participating in developmental processes, cell adhesion and motility, regulation of immune homeostasis, and recognition of glycans on the surface of viruses, bacteria and protozoan parasites. In contrast with mammals, and other vertebrate and invertebrate taxa, the identification and characterization of bona fide galectins in aquatic mollusks has been relatively recent. Most of the studies have focused on the identification and domain organization of galectin-like transcripts or proteins in diverse tissues and cell types, including hemocytes, and their expression upon environmental or infectious challenge. Lectins from the eastern oyster Crassostrea virginica, however, have been characterized in their molecular, structural and functional aspects and some notable features have become apparent in the galectin repertoire of aquatic mollusks. These including less diversified galectin repertoires and different domain organizations relative to those observed in vertebrates, carbohydrate specificity for blood group oligosaccharides, and up regulation of galectin expression by infectious challenge, a feature that supports their proposed role(s) in innate immune responses. Although galectins from some aquatic mollusks have been shown to recognize microbial pathogens and parasites and promote their phagocytosis, they can also selectively bind to phytoplankton components, suggesting that they also participate in uptake and intracellular digestion of microalgae. In addition, the experimental evidence suggests that the protozoan parasite Perkinsus marinus has co-evolved with the oyster host to be selectively recognized by the oyster hemocyte galectins over algal food or bacterial pathogens, thereby subverting the oyster's innate immune/feeding recognition mechanisms to gain entry into the host cells.

Molecular cloning, characterization, and expression analysis of two different types of lectins from the oriental river prawn, Macrobrachium nipponense

Lectins, which are widely expressed in invertebrates, play important roles in many biological processes, including protein trafficking, cell signaling, pathogen recognition, as effector molecules, and so on (Wang and Wang, 2013). This study identified one novel M-type lectin and one L-Type lectin, designated as MnMTL1 and MnLTL1, from the oriental river prawn Macrobrachium nipponense. The full-length cDNA of MnMTL1 was 2064 bp with a 1761 bp ORF encoding a putative protein of 586 deduced amino acids. The full-length cDNA of MnLTL1 was 1744 bp with a 972 bp ORF encoding a 323-amino acid peptide. The deduced MnMTL1 protein contained a putative type II transmembrane region and a 440-aa Glycoside hydrolase family 47 (GH47) domain. One luminal carbohydrate recognition domain and a 23-aa type I transmembrane region were identified from the MnLTL1. MnMTL1 shared 78% identity with Marsupenaeus japonicus M-type lectin and MnLTL1 shared 83% similarity with M. japonicus L-type lectin. RT-PCR analysis showed that MnMTL1 and MnLTL1 were expressed in all tested tissues. Quantitative real-time PCR analysis revealed that MnMTL1 and MnLTL1 are substantially fluctuant during Aeromonas hydrophila and Aeromonas veronii infections. Based on immune responses and previous literature, we assumed that MnMTL1 and MnLTL1 might be functioned as pattern recognition receptors and play important roles in the immune response of M. nipponense.

Domain repeats related to innate immunity in the South African abalone, Haliotis midae

Molluscs predominately use the cellular defence system as the primary mechanism of defence against pathogenic infection, in which haemocytes play a pivotal role. Haliotis midae is a commercially important South African species that it is susceptible to bacterial pathogens, fungal and yeast infections in the farming environment. The current study aims to enrich the current knowledge regarding H. midae innate immunity by investigating the presence and evolution of domain repeats. The bioinformatics approach used in this study, detected five repeat families in the H. midae transcriptome. These repeats families include mixed alpha and beta (leucine-rich and ankyrin), spectrin repeats, beta-propellers (WD40) and alfa-structure repeat (TPR-like). The expansion of key gene families related to host defence may be important to abalone adaptation to life in a pathogen-rich environment.

Fibrinogen-Related Proteins (FREPs) in Mollusks

Anti-parasite responses of the snail Biomphalaria glabrata involve antigen-reactive plasma lectins termed fibrinogen-related proteins (FREPs) comprising a C-terminal fibrinogen (FBG) domain and one or two upstream immunoglobulin domains. FREPs are highly polymorphic; they derive from several gene families with multiple loci and alleles that are diversified by exon loss, alternative splicing, and random somatic mutation (gene conversion and point mutations). Individual B. glabrata snails have dynamically distinct FREP sequence repertoires. The immune relevance of B. glabrata FREPs is indicated by FREP binding to polymorphic antigens of (snail-specific) digenean parasites and altered resistance of B. glabrata to digeneans following RNAi knockdown of FREPs. The compatibility polymorphism hypothesis proposes that FREP mutation increases the range of germline-encoded immune recognition in B. glabrata to counter antigenically-varied parasites. Somatic mutation may result from sequence exchange among tandemly arranged FREP genes in the genome, and analysis of sequence variants also suggests involvement of cytidine deaminase-like activity or epigenetic regulation. Without current indications of selection or retention of effective sequence variants toward immunological memory, FREP diversification is thought to afford B. glabrata immunity that is anticipatory but not adaptive. More remains to be learned about this system; other mollusks elaborate diversified lectins consisting of single FBG domains, and bona fide FREPs were reported from additional gastropod species, but these may not be diversified. Future comparative immunological studies and gene discovery driven by next-generation sequencing will further clarify taxonomic distribution of FREP diversification and the underlying mutator mechanisms as a component of immune function in mollusks.