A new Ig-superfamily member, molluscan defence molecule (MDM) from Lymnaea stagnalis, is down-regulated during parasitosis

LPS-Induced Garcia Effect and Its Pharmacological Regulation Mediated by Acetylsalicylic Acid: Behavioral and Transcriptional Evidence

Lymnaea stagnalis learns and remembers to avoid certain foods when their ingestion is followed by sickness. This rapid, taste-specific, and long-lasting aversion-known as the Garcia effect-can be formed by exposing snails to a novel taste and 1 h later injecting them with lipopolysaccharide (LPS). However, the exposure of snails to acetylsalicylic acid (ASA) for 1 h before the LPS injection, prevents both the LPS-induced sickness state and the Garcia effect. Here, we investigated novel aspects of this unique form of conditioned taste aversion and its pharmacological regulation. We first explored the transcriptional effects in the snails' central nervous system induced by the injection with LPS (25 mg), the exposure to ASA (900 nM), as well as their combined presentation in untrained snails. Then, we investigated the behavioral and molecular mechanisms underlying the LPS-induced Garcia effect and its pharmacological regulation by ASA. LPS injection, both alone and during the Garcia effect procedure, upregulated the expression levels of immune- and stress-related targets. This upregulation was prevented by pre-exposure to ASA. While LPS alone did not affect the expression levels of neuroplasticity genes, its combination with the conditioning procedure resulted in their significant upregulation and memory formation for the Garcia effect.

Transcriptomic Profile of the Cockle Cerastoderma edule Exposed to Seasonal Diarrhetic Shellfish Toxin Contamination

Bivalves constitute an important source of proteins for human consumption, but some accumulate biotoxins such as diarrhetic shellfish toxins (DSTs), constituting a risk to human health. The cockle Cerastoderma edule is one of the most important species harvested in the Portuguese coast but also one of the most affected species due to recurrent DSTs exposure. However, little is known regarding the effects of the toxins produced by blooming dinoflagellates on C. edule. Herein, we explore the Differentially Expressed Genes (DEGs) of two tissues (gills and digestive gland) from wild cockles sampled in Portugal, through their whole transcriptomic response in two different seasons (exposed and not exposed to DSTs). The de novo transcriptome assembly returned 684,723 contigs, N50 of 1049, and 98.53% completeness. Altogether, 1098 DEGs were identified, of which 353 DEGs were exclusive for the digestive gland, 536 unique for the gills and 209 DEGs were common. Among DEGs were identified known DSTs-biomarkers including glutathione peroxidase, glutathione S-transferase, superoxide dismutase, cytochrome P450, ABC transporters, actin and tubulin-related proteins, Heat shock proteins and complement C1Q-like proteins. This study provides the first transcriptomic profile of C. edule, giving new insights about its molecular responses under different environmental conditions of DSTs exposure.

The unlimited potential of the great pond snail, Lymnaea stagnalis

Only a limited number of animal species lend themselves to becoming model organisms in multiple biological disciplines: one of these is the great pond snail, Lymnaea stagnalis. Extensively used since the 1970s to study fundamental mechanisms in neurobiology, the value of this freshwater snail has been also recognised in fields as diverse as host-parasite interactions, ecotoxicology, evolution, genome editing and 'omics', and human disease modelling. While there is knowledge about the natural history of this species, what is currently lacking is an integration of findings from the laboratory and the field. With this in mind, this article aims to summarise the applicability of L. stagnalis and points out that this multipurpose model organism is an excellent, contemporary choice for addressing a large range of different biological questions, problems and phenomena.

Snail defence responses to parasite infection: The Lymnaea stagnalis-Trichobilharzia szidati model

Lymnaea stagnalis is a common freshwater gastropod. Importantly, the snail serves as the intermediate host for more than one hundred species of digenetic trematodes, including the avian schistosome Trichobilharzia szidati, a causative agent of cercarial dermatitis in humans. Infection of L. stagnalis by T. szidati initiates a dynamic confrontation between the host and the parasite that culminates in immunocompatibility ensuring survival and development of larvae. Unfortunately, the molecular mechanisms determining this immunocompatibility remain poorly characterised. By employing a variety of immune elicitors, including chemical compounds, PAMPs and bacteria, research in the last two decades has elucidated some of the molecular processes that regulate the snail internal defence response such as haemocyte signalling pathways. These discoveries provide a framework for future studies of molecular interactions between T. szidati and L. stagnalis to help elucidate factors and mechanisms enabling transmission of schistosome parasites. Moreover, support from recently available next generation sequence data and CRISPR-enabled functional genomics should further enable L. stagnalis as an important model for comparative immunology and contribute to a more comprehensive understanding of immune functions in gastropod molluscs.

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.

Avian schistosomes and outbreaks of cercarial dermatitis

Cercarial dermatitis (swimmer's itch) is a condition caused by infective larvae (cercariae) of a species-rich group of mammalian and avian schistosomes. Over the last decade, it has been reported in areas that previously had few or no cases of dermatitis and is thus considered an emerging disease. It is obvious that avian schistosomes are responsible for the majority of reported dermatitis outbreaks around the world, and thus they are the primary focus of this review. Although they infect humans, they do not mature and usually die in the skin. Experimental infections of avian schistosomes in mice show that in previously exposed hosts, there is a strong skin immune reaction that kills the schistosome. However, penetration of larvae into naive mice can result in temporary migration from the skin. This is of particular interest because the worms are able to migrate to different organs, for example, the lungs in the case of visceral schistosomes and the central nervous system in the case of nasal schistosomes. The risk of such migration and accompanying disorders needs to be clarified for humans and animals of interest (e.g., dogs). Herein we compiled the most comprehensive review of the diversity, immunology, and epidemiology of avian schistosomes causing cercarial dermatitis.

Characterization of a novel hemolytic activity of human IgG fractions arising from diversity in protein and oligosaccharide components

Human IgG is a well-established multifunctional antigen specific immunoglobulin molecule of the adaptive immune system. However, an antigen nonspecific immunological function of human IgG has never been reported. In this study, human IgG was isolated using ammonium sulfate fractional precipitation and diethylaminoethanol (DEAE) cellulose 52 ion exchange chromatography, from which h-IgG and hs-IgG fractions were purified on the basis of their differential binding to rabbit anti-shrimp hemocyanin antibody (h) and rabbit anti-shrimp hemocyanin's small subunit antibody (hs), respectively. We found that h-IgG had a higher hemolytic activity than hs-IgG against erythrocytes from humans, rabbits, mice and chickens, whereas the control IgG showed negligible activity. h-IgG could interact directly with erythrocyte membranes, and this interaction was suppressed by high molecular weight osmoprotectants, showing that it may follow a colloid-osmotic mechanism. In comparative proteomics and glycomics studies, h-IgG and hs-IgG yielded 20 and 5 significantly altered protein spots, respectively, on a 2-D gel. The mean carbohydrate content of h-IgG and hs-IgG was approximately 3.6- and 2-fold higher than that of IgG, respectively, and the α-d-mannose/α-d-glucose content was in the order of h-IgG>hs-IgG>IgG. In this study, a novel antigen nonspecific immune property of human IgG was investigated, and the diversity in the protein constituents and glycosylation levels may have functional signficance.

Effects of short-term exposure to environmentally relevant concentrations of different pharmaceutical mixtures on the immune response of the pond snail Lymnaea stagnalis

Pharmaceuticals are pollutants of potential concern in the aquatic environment where they are commonly introduced as complex mixtures via municipal effluents. Many reports underline the effects of pharmaceuticals on immune system of non target species. Four drug mixtures were tested, and regrouped pharmaceuticals by main therapeutic use: psychiatric (venlafaxine, carbamazepine, diazepam), antibiotic (ciprofloxacine, erythromycin, novobiocin, oxytetracycline, sulfamethoxazole, trimethoprim), hypolipemic (atorvastatin, gemfibrozil, benzafibrate) and antihypertensive (atenolol, furosemide, hydrochlorothiazide, lisinopril). Their effects were then compared with a treated municipal effluent known for its contamination, and its effects on the immune response of Lymnaea stagnalis. Adult L. stagnalis were exposed for 3 days to an environmentally relevant concentration of the four mixtures individually and as a global mixture. Effects on immunocompetence (hemocyte viability and count, ROS and thiol levels, phagocytosis) and gene expression were related to the immune response and oxidative stress: catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GR), Selenium-dependent glutathione peroxidase (SeGPx), two isoforms of the nitric oxide synthetase gene (NOS1 and NOS2), molluscan defensive molecule (MDM), Toll-like receptor 4 (TLR4), allograft inflammatory factor-1 (AIF) and heat-shock protein 70 (HSP70). Immunocompetence was differently affected by the therapeutic class mixtures compared to the global mixture, which increased hemocyte count, ROS levels and phagocytosis, and decreased intracellular thiol levels. TLR4 gene expression was the most strongly increased, especially by psychiatric mixture (19-fold), while AIF-1, GR and CAT genes were downregulated. A decision tree analysis revealed that the immunotoxic responses caused by the municipal effluent were comparable to those obtained with the global pharmaceutical mixture, and the latter shared similarity with the antibiotic mixture. This suggests that pharmaceutical mixtures in municipal effluents represent a risk for gastropods at the immunocompetence levels and the antibiotic group could represent a model therapeutic class for municipal effluent toxicity studies in L. stagnalis.

Immunotoxicity of surface waters contaminated by municipal effluents to the snail Lymnaea stagnalis

The immunotoxic effects of surface waters contaminated by a municipal effluent dispersion plume were examined in the snail Lymnaea stagnalis. Snails were exposed to surface waters where changes in hemocyte counts, viability, levels of reactive oxygen species (ROS), reduced thiols and phagocytic activity were tracked following exposure periods of 3h and 3 and 7d. Changes in mRNA expression of some genes in the hemocytes were also assessed after 7d of exposure, as follows: genes coding for catalase (CAT), superoxide dismutase (SOD), glutathione reductase (GSR), selenium-dependent glutathione peroxidase (SeGPX), two isoforms of the nitric oxide synthetase (NOS1 and NOS2), molluscan defensive molecule (MDM), toll-like receptor 4 (TLR4), allograft inflammatory factor-1 (AIF), and heat-shock protein 70 (HSP70). At the sites closest to the discharge point, exposure led to impaired hemocyte viability and intracellular thiol levels and also an increase of hemocyte count, ROS levels and phagocytosis. Phagocytosis and ROS levels in hemocytes were correlated with heterotrophic bacterial counts in snails. We found four genes with increased mRNA expression as a response to exposure of municipal wastewaters: TLR4 (6-fold), HSP70 (2-fold), SeGPx (4-fold) and CAT (2-fold). Immunocompetence responses were analyzed by canonical analysis to seek out relationships with mRNA expression of the genes involved in stress, pattern recognition, cellular and humoral responses. The data revealed that genes involved in oxidative stress were strongly involved with immunocompetence and that the resulting immune responses were influenced both by the bacterial and pollutant loadings of the effluent.

Immunoglobulin superfamily is conserved but evolved rapidly and is active in the silkworm, Bombyx mori

Immunoglobulin superfamily (IgSF) proteins are known for their abilities to specifically recognize and adhere to cells. In this paper, we predicted the presence of 133 IgSF proteins in the silkworm (Bombyx mori) genome. Comparison with similar proteins in other model organisms (Caenorhabditis elegans, Drosophila melanogaster, Anopheles gambiae, Apis mellifera and Homo sapiens) indicated that IgSF proteins are conserved but have rapidly evolved from worms to human beings. However, these proteins are well conserved amongst insects. Silkworm microarray-based expression data showed tissue expression of 57 IgSF genes and microbe-induced differential expression of 37 genes. Based on the expression data, we can conclude that the silkworm IgSF is active.

Expression profiling and binding properties of fibrinogen-related proteins (FREPs), plasma proteins from the schistosome snail host Biomphalaria glabrata

A growing body of evidence suggests an important role for fibrinogen-like proteins in innate immunity in both vertebrates and invertebrates. It has been shown that fibrinogen-related proteins (FREPs), plasma proteins present in the freshwater snail Biomphalaria glabrata, the intermediate host for the human blood fluke Schistosoma mansoni, are diverse and involved in snail innate defense responses. To gain further insight into the functions of FREPs, recombinant FREP proteins (rFREPs) were produced in Escherichia coli and antibodies (Abs) were raised against the corresponding rFREPs. We first show that most FREP proteins exist in their native conformation in snail hemolymph as multimeric proteins. Western blot analyses reveal that expression of multiple FREPs including FREP4 in plasma from M line and BS-90 snails, which are susceptible and resistant to S. mansoni infection, respectively, is up-regulated significantly after infection with the trematode Echinostoma paraensei. Moreover, our assays demonstrate that FREPs are able to bind E. paraensei sporocysts and their secretory/excretory products (SEPs), and a variety of microbes (Gram-positive and Gram-negative bacteria and yeast). Furthermore, this binding capability shows evidence of specificity with respect to pathogen type; for example, 65-75-kDa FREPs (mainly FREP4) bind to E. paraensei sporocysts and their SEPs whereas 95-kDa and 125-kDa FREPs bind the microbes assayed. Our results suggest that FREPs can recognize a wide range of pathogens, from prokaryotes to eukaryotes, and different categories of FREPs seem to exhibit functional specialization with respect to the pathogen encountered.

Immunoglobulin superfamily members play an important role in the mosquito immune system

Immunoglobulin superfamily (IgSF) proteins are known for their ability to specifically recognize and adhere to other molecules, mediating cell-surface reception and pathogen recognition. Mammalian IgSF proteins such as antibodies are among the best characterized molecules of the immune system; in contrast, the involvement of invertebrate IgSF members in immunity has not been broadly studied. Analysis of the predicted Anopheles gambiae transcriptome identified 138 proteins that have at least one immunoglobulin domain. Challenge with Plasmodium, Gram-negative or Gram-positive bacteria resulted in significant regulation of 85 IgSF genes, indicating potential roles for these molecules in infection responses and immunity. Based on sequence and expression data, six infection-responsive with immunoglobulin domain (IRID 1-6) genes were chosen and functionally characterized with regard to their role in innate immunity. Reverse-genetic gene-silencing assays showed IRID3, IRID5 and IRID6 contribute to viability upon bacterial infection while IRID4 and IRID6 are involved in limiting Plasmodium falciparum infection.

Stress and immune responses in abalone: limitations in current knowledge and investigative methods based on other models

Increasing mariculture of abalone focuses attention on their immune and stress responses. For abalone, as well as many invertebrates, the function and relationship of these systems and how in vitro tests relate to them are not fully understood. This review focuses on research into the immune system and stress response conducted on abalone and on aspects that can be monitored in vitro. To fill the considerable knowledge gaps, we discuss work on other invertebrate taxa, concentrating on those closest to abalone, and making explicit the phylogenetic relations involved. The stress response appears to be very similar to that in vertebrates, but interpreting most immune responses remains problematic. Phylogeny must be considered: immune function tests derived from research into vertebrates or distantly related invertebrates should not be used in abalone until they have been validated in abalone by studies of susceptibility to pathogens. We suggest phagocytic activity of haemocytes and their efficiency in clearing bacteria are reliable parameters to measure, because they have been directly related to immune competency and are consistently depressed by stress. Carefully designed assays of antimicrobial activity may also be useful. Important aims of future research will be to investigate the relationship between growth, stress and robust immunity, and to develop tests that can be run on production animals, which accurately depict immune status.

Ancient Phylogenetic Beginnings of Immunoglobulin Hypermutation

Many structures and molecules closely related to those involved in the specific process of immunoglobulin (Ig) hypermutation existed before the appearance of primordial Ig genes. Consequently, these structures can be found even in animals and organisms distinct from vertebrates; likewise, homologues of hypermutation enzymes are present in a broad range of species, from bacteria to mammals. Our analysis, based predominantly on primary structure, demonstrates the existence of molecules similar to Ig domains, variable Ig domains (IGv), and antigen receptors (AR) in unicellular organisms, nonvertebrate metazoans, and nonvertebrate Coelomata, respectively. In addition, we deal here with some important structural properties of CDR1-like segments of the selected sponge adhesion molecule GCSAMS exhibiting chimerical Ig domain similarities, and demonstrate the occurrence of conserved regions corresponding to Ohno's modern intact primordial building block in the C-terminal part of IGv-related segments of nonvertebrate origin. The results of our analysis are also discussed with respect to the possible phylogeny of molecules preceding the hypothetical common antigen receptor ancestor.

Do trematode parasites disrupt defence-cell signalling in their snail hosts?

More than a decade ago, it was postulated that components derived from trematode parasites block receptors on the defence cells of their snail intermediate hosts, thus preventing host-cell activation and parasite elimination. This phenomenon has still not been investigated extensively. However, recent work concerning the molecular regulation of the molluscan defence response provides a new framework for studies that focus on an extension of this original concept - subversion of host cell signalling by trematode parasites. The hypothesis is that, to facilitate survival and replication in their intermediate hosts, trematode parasites down regulate host defence responses by interfering with key signal-transduction pathways in snail defence cells.

Molluscan and vertebrate immune responses to bird schistosomes

There is a growing understanding of risks posed by human contact with the cercariae of bird schistosomes. In general, there are no fundamental biological differences between human and bird schistosomes in terms of their interactions with snail and vertebrate hosts. The penetration of host surfaces is accompanied by the release of penetration gland products and the shedding of highly antigenic surface components (miracidial ciliated plates and cercarial glycocalyx) which trigger host immune reactions. New surface structures are formed during transformation: the tegument of mother sporocysts and the tegumental double membrane of schistosomula. These surfaces apparently serve as protection against the host immune response. Certain parasite excretory-secretory products may contribute to immunosuppression or, on the other hand, stimulation of host immune reactions. Discovery of new species and their life cycles, the characterization of host-parasite interactions (including at the molecular level), the determination of parasite pathogenicity towards the host, the development of tools for differential diagnosis and the application of protective measures are all topical research streams of the future. Regularly updated information on bird schistosomes and cercarial dermatitis can be found at http://www.schistosomes.cz (web pages of Schistosome Group Prague).

Representation of an immune responsive gene family encoding fibrinogen-related proteins in the freshwater mollusc Biomphalaria glabrata, an intermediate host for Schistosoma mansoni

Fibrinogen-related proteins (FREPs) are found in the hemolymph of the freshwater snail Biomphalaria glabrata, are up-regulated following exposure to digenetic trematode parasites, and bind to trematode larval surfaces, suggestive of a role in internal defense. Southern blot and degenerate-polymerase chain reaction (PCR) analyses were undertaken to better understand the diversity of the FREP-encoding gene family. Probes corresponding to the N-terminal IgSF domains of specific FREP gene subfamilies (FREPs 2, 3, 4, 7, 12 and 13) revealed between 1 to 8 loci per subfamily on Southern blots. Probes representing the relatively conserved C-terminal fibrinogen domain of FREPs bound many sequences in Southern blots of genomic DNA from B. glabrata, and from two related gastropod species, Biomphalaria pfeifferi and Helisoma trivolvis. Using degenerate-PCR, we obtained 42 unique fibrinogen-encoding sequences from 180 clones derived from a single individual of the M-line strain of B. glabrata, further supporting the notion of their abundant representation in the B. glabrata genome. The fibrinogen-encoding sequences of FREPs encoding one or two IgSF domains tended to separate into distinct clades, but bootstrap support for this separation was low. A novel category of fibrinogen-encoding sequence was also revealed. This study provides the approximate number of gene copies in several FREP subfamilies, confirms the existence of a diverse FREP gene family, reports additional unusual sequences encoding fibrinogen-like molecules, and provides further justification to explore the functional roles of FREPs in both B. glabrata and B. pfeifferi, both important intermediate hosts of the human pathogen, Schistosoma mansoni.

Granularin, a novel molluscan opsonin comprising a single vWF type C domain is up‐regulated during parasitation

Snails are intermediate hosts to schistosome parasites, some of which are the main cause of human schistosomiasis (bilharzia), and have been used as models for parasite-host interactions for a long time. Here, we have characterized a novel internal defense peptide of the snail Lymnaea stagnalis, of which the relative abundance in brain tissue increases upon infection with the avian schistosome Trichobilharzia ocellata. This protein, named granularin, is secreted by granular cells, which are numerous in the connective tissue surrounding the brain. The protein is unique because it comprises only a single Von Willebrand factor type C domain that is normally found in large transmembrane and secreted extracellular matrix proteins. The granularin gene is twice up-regulated during parasitation. Purified granularin stimulates phagocytosis of foreign particles by blood hemocytes. Together, our data indicate that granularin represents a novel protein that acts as an opsonin in the molluscan internal defense response.

Trematodes and snails: an intimate association

Trematode parasites share an intimate relationship with their gastropod intermediate hosts, which act as the vehicle for their development and transmission. They represent an enormous economic and medical burden in developing countries, stimulating much study of snail–trematode interactions. Laboratory-maintained snail–trematode systems and in vitro cell cultures are being used to investigate the molecular dialogue between host and parasite. These dynamic and finely balanced antagonistic relationships, in which parasites strongly influence the physiology of the host, are highly specific and may occasionally demonstrate co-speciation. We consider the mechanisms and responses deployed by trematodes and snails that result in compatibility or rejection of the parasite, and the macroevolutionary implications that they may effect. Although for gastropods the fossil record gives some insight into evolutionary history, elucidation of trematode evolution must rely largely upon molecular approaches, and for both, such techniques have provided fresh and often surprising evidence of their origins and dispersal over time. Co-evolution of snails and trematodes is becoming increasingly apparent at both cellular and population levels; the implications of which are only beginning to be understood for disease control. Untangling the complex interactions of trematodes and snails promise fresh opportunities for intervention to relieve the burden of parasitic disease.

The FREP gene family in the snail Biomphalaria glabrata: additional members, and evidence consistent with alternative splicing and FREP retrosequences. Fibrinogen-related proteins

Fibrinogen-related proteins (FREPs) found in hemolymph of the snail Biomphalara glabrata are hypothesized to be involved in non-self recognition. Among 150 cloned FREP cDNAs examined, we have identified three additional FREP members, FREPs 3.3, 12.1 and 13.1, bringing the total of FREP subfamilies to 13. The new FREPs each encode two immunoglobulin superfamily domains and a fibrinogen domain. Additionally, five truncated cDNAs with >99% nucleotide identity in coding regions to FREPs 3.2, 12.1 or 13.1 were identified. The truncated forms, the first reported for FREPs, lack a partial exon, one complete exon, or two complete exons plus the 3'UTR. Our preferred hypothesis is that all five truncated cDNAs observed arise from alternative splicing of full-length FREP genes. Genomic sequences lacking at least two introns and corresponding to the 3' ends of the cDNAs of FREP12.1 and its two truncated forms were also recovered. Although these could be the source of the truncated cDNAs, they are believed to be retrosequences.

Biology of the schistosome genus Trichobilharzia

Trichobilharzia is the largest genus within the family Schistosomatidae, covering over 40 species of avian parasites. To clarify the existing confusion in the systematics of the genus, we recommend combining knowledge of life cycles and developmental stages, snail/bird hosts, cytogenetical and molecular data together with morphological criteria for the characterization of particular species. The high specificity of Trichobilharzia for the intermediate host is a likely reflection of the ability to avoid the internal defence of specific snails. The spectrum of final hosts (birds) seems to be much wider. The infection of birds--trichobilharziasis--may lead to considerable tissue injuries, caused by eggs of the parasite or migration of immature/mature worms through the body. Most Trichobilharzia (visceral species) migrate through the viscera of the host, but nasal species display a neurotropic mode of migration. Due to a low specificity of penetrating cercariae, mammals (including humans) can be attacked. This leads to cercarial dermatitis, predominantly in sensitized hosts. Experimental infections indicate that Trichobilharzia never mature in an incompatible (mammalian) host. However, not all cercariae and schistosomula are necessarily trapped and eliminated in the skin, and parasites may migrate throughout the viscera and the nervous system of mammals. These findings suggest that the pathogenicity of Trichobilharzia may have been underestimated in the past and health risks associated with trichobilharziasis need to be studied further.

Molecular studies of the molluscan response to digenean infection

A classical body of knowledge regarding molluscan immunobiology has developed over the years that indicates an important role for both soluble hemolymph proteins and hemocytes in internal defense. The incorporation of powerful molecular approaches into the toolkit of the molluscan immunobiologist holds considerable promise for allowing a more precise description of the molecules and processes involved. Recent discoveries indicate that the molluscan internal defense system has components that are simultaneously both tantalizingly similar to, and different from, molecules functioning in a defense context in other phyla. Study of molecular components of the internal defense systems of molluscs promises to be a rewarding and surprising field.

Invertebrate molecular neuroimmune processes

During the course of evolution, invertebrates and vertebrates have kept in common similar signaling molecules e.g. neuropeptides, opiates etc... Complete hormonal-enzymatic systems such as the opioid-opiate-cannabinoid systems have been found in both nervous central and immune systems of these animals. These signaling molecules can be found free in blood circulation and act as immunomodulators. The present review is focused on peptides derived from the opioid proopiomelanocortin precursor, the opiates and the endocannabinoids, which are very powerful immunosuppressors, and example models of the bidirectional communications between the endocrine and the immune systems. Parasites use these immunosuppressors with magnificence in their crosstalk with their host.

Regeneration as an application of gastropod neural plasticity

Gastropod research is providing many insights into mechanisms of neural regeneration. These observations were made possible by the pioneering work of individuals who described the nervous systems of gastropods, mapped prominent neurons and determined their roles and connections, and developed the techniques for culturing them. This information has allowed questions about injury responses, target selection, and pathway cues to be explored at the level of individually identified neurons. Because of gastropod studies, more is known about axon sealing, growth cone formation and behavior, signals that travel from the site of axotomy to the soma, and the second messengers that are activated there. The responses in neurons and non-neuronal cells during neural development and injury are coordinated by chemical messenger systems that are highly conserved, including neurotransmitters, cytokines, and neurotrophins. The nervous system is modified in learning paradigms by some of the same messenger systems activated by injury, because learning and injury both challenge neurons to change. The conservation of basic mechanisms that coordinate neuronal plasticity allows us to approach basic questions in relatively simple nervous systems with reasonable confidence that the findings will be relevant for other nervous systems, including possible applications to the mammalian nervous system.

Parasites flicking the NPY gene on the host's switchboard: why NPY?

It was investigated whether up-regulation of the NPY gene by the schistosome Trichobilharzia ocellata in its snail host Lymnaea stagnalis redirects the host's energy flows. We cloned the cDNA encoding Lymnaea NPY (LyNPY), purified and sequenced the peptide, and used synthesized peptide for physiological and morphological studies. Increasing the LyNPY titer in nonparasitized snails (mimicking parasitosis) by 1) implantation of slow-release pellets and 2) injections suppressed reproductive activity and reduced growth in a dose- and time-dependent manner without affecting food intake. When the LyNPY titer was back to normal, reproduction and growth were resumed, coinciding with a transient increase of food intake serving to replenish glycogen stores. Observations on double-immunostained whole mount preparations of brains support these data. A close association was found between LyNPY-positive axons and axons both from ovulation hormone-producing neurons and molluscan insulin-like peptide-producing neurons involved in regulation of growth. As no synaptic(-like) contacts were observed, it is supposed that LyNPY acts nonsynaptically. No morphological interaction was found between LyNPY-positive axons and motoneurons innervating the feeding apparatus. Our data explain why it is an advantageous strategy for endoparasites to up-regulate the highly conserved NPY gene in their host.

Cell adhesion molecules in invertebrate immunity

Cell adhesion is essential in immunity in invertebrates, e.g., in the cellular immune responses of encapsulation and nodule formation. Here cell adhesion molecules shown or suggested to be involved in invertebrate immunity are reviewed. Blood cells of the crayfish, Pacifastacus leniusculus, can release a cell-adhesive and opsonic peroxidase, peroxinectin. A site containing the motif, KGD, appears to be adhesive by binding to a transmembrane receptor of the integrin family on the blood cells. Peroxinectin also binds a peripheral blood cell surface CuZn-superoxide dismutase. The peroxidase-integrin interaction appears to have evolved early and seems conserved; human myeloperoxidase supports cell adhesion via the alphaMbeta2 integrin. There is evidence for peroxinectin-like proteins in other arthropods. Effects by RGD peptides indicate that integrins mediate blood cell adhesion and cellular immunity in diverse invertebrate species. Other invertebrate blood cell molecules proposed to be involved in adhesion include the insect plasmatocyte-spreading peptide, as well as soluble and transmembrane proteins which show some similarity to vertebrate adhesive or extracellular matrix molecules. Proteins such as the Ig family member hemolin, or proteins found in insects that are hosts for parasitic wasps, inhibit cell adhesion and may regulate or block cellular immunity.

vig-1, a new fish gene induced by the rhabdovirus glycoprotein, has a virus-induced homologue in humans and shares conserved motifs with the MoaA family

We used mRNA differential display methodology to analyze the shift of transcription profile induced by the fish rhabdovirus, viral hemorrhagic septicemia virus (VHSV), in rainbow trout leukocytes. We identified and characterized a new gene which is directly induced by VHSV. This VHSV-induced gene (vig-1) encodes a 348-amino-acid protein. vig-1 is highly expressed during the experimental disease in lymphoid organs of the infected fish. Intramuscular injection of a plasmid vector expressing the viral glycoprotein results in vig-1 expression, showing that the external virus protein is sufficient for the induction. vig-1 expression is also obtained by a rainbow trout interferon-like factor, indicating that vig-1 can be induced through different pathways. Moreover, vig-1 is homologous to a recently described human cytomegalovirus-induced gene. Accordingly, vig-1 activation may represent a new virus-induced activation pathway highly conserved in vertebrates. The deduced amino acid sequence of vig-1 is significantly related to sequences required for the biosynthesis of metal cofactors. This suggests that the function of vig-1 may be involved in the nonspecific virus-induced synthesis of enzymatic cofactors of the nitric oxide pathway.

Role of the prophenoloxidase-activating system in invertebrate immunity

The melanization reaction, which is a common response to parasite entry in invertebrate animals, especially arthropods, is due to the activity of an oxidoreductase, phenoloxidase. This enzyme is part of a complex system of proteinases, pattern recognition proteins and proteinase inhibitors constituting the so-called prophenoloxidase-activating system. It is proposed to be a non-self recognition system because conversion of prophenoloxidase to active enzyme can be brought about by minuscule amounts of molecules such as lipopolysaccharide, peptidoglycan and beta-1, 3-glucans from micro-organisms. Several components of this system recently have been isolated and their structure determined.

Cell adhesion properties of hemolin, an insect immune protein in the Ig superfamily

The isolation of antibacterial peptides from the giant silkmoth Hyalophora cecropia has opened the area of animal antibiotics [Boman, H. G. (1991) Cell 65, 205-207] and the study of insect immune genes has revealed striking similarities to many immune response genes in mammals [Hultmark, D. (1994) Nature 267, 116-117]. However, the molecules and mechanisms behind primordial immune recognition are not understood. One candidate for one such recognition molecule is hemolin, a 48-kDa immunoglobulin-related protein first isolated from H. cecropia, where it is up-regulated upon infection and secreted into the hemolymph. Hemolin was shown to bind to bacteria and to hemocytes, giving rise to changes in hemocyte adhesiveness and intracellular phosphorylation patterns [Faye, I. & Kanost, M. (1997) in Molecular mechanisms of immune responses in insects (Brey, P. T. & Hultmark, D., eds) Chapman and Hall, London]. In the present publication, we give evidence for the presence of a 52-kDa membrane form of hemolin on hemocytes, based on flow-activated cell sorting and membrane protein extractions. In addition we reveal calcium-dependent homophilic binding properties of hemolin, using hemolin-coated microspheres. When biotinylated recombinant hemolin was allowed to bind to hemocyte membranes, higher molecular-mass complexes were formed. Furthermore, we used immunological methods and Northern-blot analysis to demonstrate the presence of hemolin in embryos and retinal discs, suggesting that hemolin is expressed in several tissues at different developmental stages. These results show novel cell adhesion features of hemolin, corroborating its multifunctional character with putative roles in cellular and humoral immunity and in development.

Altered gene expression in the host brain caused by a trematode parasite: neuropeptide genes are preferentially affected during parasitosis

Schistosome parasites adjust the physiology and behavior of their intermediate molluscan hosts to their own benefit. Previous studies demonstrated effects of the avian-schistosome Trichobilharzia ocellata on peptidergic centers in the brain of the intermediate snail host Lymnaea stagnalis. In particular, electrophysiological properties and peptide release of growth- and reproduction-controlling neuroendocrine neurons were affected. We now have examined the possibility that the expression of genes that control physiology and behavior of the host might be altered during parasitosis. A cDNA library of the brain of parasitized Lymnaea was constructed and differentially screened by using mRNA from the brain of both parasitized and nonparasitized snails. This screening yielded a number of clones, including previously identified cDNAs as well as novel neuronal transcripts, which appear to be differentially regulated. The majority of these transcripts encode neuropeptides. Reverse Northern blot analysis confirmed that neuropeptide gene expression is indeed affected in parasitized animals. Moreover, the expression profiles of 10 transcripts tested showed a differential, parasitic stage-specific regulation. Changes in expression could in many cases already be observed between 1.5 and 5 hr postinfection, suggesting that changes in gene expression are a direct effect of parasitosis. We suggest that direct regulation of neuropeptide gene expression is a strategy of parasites to induce physiological and behavioral changes in the host.