Digenean-gastropod host associations inform on aspects of specific immunity in snails

Histopathological Lesions Caused by a Digenean Trematode in a Pest Apple Snail, Pomacea canaliculata, in Its Native Geographic Distribution Area

Pomacea canaliculata is one of the most dangerous invasive species. Morphological and molecular analyses have revealed that a digenean species belonging to the family Echinostomatidae parasitizes this snail at two sites in Buenos Aires Province, Argentina, South America. Molecular results confirmed that the species belongs to a genus closely related to Patagifer. Analysis of the 28S rDNA showed that the sequences of the rediae and metacercariae are identical, indicating that the apple snail acts as the first and second intermediate host. The cercariae may encyst as metacercaria inside the redia and also emerge and re-infect the same snail or another snail. The prevalence of digeneans was higher in one of the sampling locations (15.1% vs. 0.72%), probably because the bird species that acts as the definitive host is more abundant in that area. Histopathological examination showed that the parasite quickly invades multiple host organs (gills, intestines, albumen gland, lung, kidney, and mantle border) besides the gonad and digestive gland, as is usual in digeneans. In addition, the partial or total castration of snails was observed in cases of moderate and high infection intensity. In males, there was loss of integrity in testicular tubules, while in females, the replacement of ovarian tissue by rediae was found.

Immuno-molecular profile for Biomphalaria glabrata/Schistosoma mansoni interaction

The complex innate immune defense of Biomphalaria glabrata, the intermediate host of Schistosoma mansoni, governs the successful development of the intramolluscan stages of the parasite. The interaction between the snail and the parasite involves a complex immune molecular crosstalk between several parasite antigens and the snail immune recognition receptors, evoking different signals and effector molecules. This work seeks to discuss the immune-related molecules that influence compatibility in Biomphalaria glabrata/Schistosoma mansoni interaction and the differential expression of these molecules between resistant and susceptible snails. It also includes the current understanding of the immune molecular determinants that govern the compatibility in sympatric and allopatric interactions, and the expression of these molecules after immune priming and the secondary immune response. Herein, the differences in the immune-related molecules in the interaction of other Biomphalaria species with Schistosoma mansoni compared to the Biomphalaria glabrata model snail are highlighted. Understanding the diverse immune molecular determinants in the snail/schistosome interaction can lead to alternative control strategies for schistosomiasis.

Investigating the internal system of defense of Gastropoda Aplysia depilans (Gmelin, 1791): Focus on hemocytes

The internal defense system of mollusks represents an efficient protection against pathogens and parasites, involving several biological immune processes, such as phagocytosis, encapsulation, cytotoxicity, and antigenic recognition of self/non-self. Mollusks possess professional, migratory, and circulating cells that play a key role in the defense of the organism, the hemocytes. Several studies have been performed on hemocytes from different mollusks, but, to date, these cells are still scarcely explored. Different hemocyte populations have been found, according to the presence or absence of granules, size, and the species of mollusks studied. Our study aims to deepen the knowledge of the hemocytes of the gastropod Aplysia depilans using morphological techniques and light and confocal microscopy, testing Toll-like receptor 2, inducible nitric oxide synthetase, and nicotinic acetylcholine receptor alpha 7 subunit. Our results show two hemocyte populations distinguishable by size, and presence/absence of granules in the cytoplasm, strongly positive for the antibodies tested, suggesting for the first time the presence of these receptors on the surface of sea hare hemocytes by immunohistochemistry. These data help in the understanding of the immune system of this gastropod, providing additional data for comprehending the evolution of the defense response in metazoan phylogenesis.

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.

Land snails can trap trematode cercariae in their shell: Encapsulation as a general response against parasites?

Terrestrial gastropods are hosts of a wide variety of metazoan parasites and can respond to parasite exposure in various ways. One of these defence mechanisms, the ability to trap parasites in the host shell, was previously thought to apply only against nematodes. During a field survey along an urbanisation gradient, we found that the shell of Cornu aspersum and Cepaea nemoralis can contain encapsulated trematode cercariae, with prevalences of 7% and 1%, respectively over the entire sample, and up to 47% at the local population level. To our knowledge, this is the first case study unambiguously showing that land snails can trap non-nematode parasites in their shell at non-negligible prevalences. Shell-encapsulation could be a more general defence mechanism than previously described, and more studies are needed to understand its importance and variability.

Structural insights into the redox regulation of Oncomelania hupensis TRP14 and its potential role in the snail host response to parasite invasion

The freshwater amphibious snail Oncomelania hupensis is the unique intermediate host of Schistosoma japonicum, but little attention has been paid to the interaction between the two. In snails, the production of reactive oxygen species (ROS) by hemocytes has been shown to be vital for snail immune defense against schistosome infection. However, excessive ROS accumulation could lead to oxidative damage, requiring the antioxidant system for maintaining the cellular redox homeostasis. Previously we identified a thioredoxin-related protein of 14 kDa from O. hupensis (OhTRP14), and showed that it was involved in the scavenging of ROS in circulating hemocytes. Here, we confirmed that OhTRP14 plays a potential role in the snail host response to parasite challenge and determined the crystal structures of OhTRP14 in two different states (oxidized and transition state). The overall structure revealed a typical Trx fold and is similar to that of human TRP14 (hTRP14), but there were significant structural differences between the two states. Noticeably, there was a different pair of thiol groups from Cys30 and Cys44 in the transition state of OhTRP14, were with the similar separation of 2.9 Å as that (2.6 Å) between Cys41 and Cys44, but in a different orientation, suggesting that the Cys30 is likely to function as an important molecular switch involved in the oxidoreductase activity of OhTRP14. Comparative studies between OhTRP14 and hTRP14 by analyzing the surface characteristics, charge distribution and oxidoreductase activity toward insulin demonstrated they might have similar substrates. The results are expected to provide structural insights into the redox regulation of OhTRP14 and contribute to better understanding of TRP14 family. DATA DEPOSITION: The atomic coordinates of the structure and the structure factors were deposited in Protein Data Bank with PDB ID codes 7XQ3 and 7XPW.

Molecular characterization of thioester-containing proteins in Biomphalaria glabrata and their differential gene expression upon Schistosoma mansoni exposure

Schistosomiasis is a disease caused by trematode parasites of the genus Schistosoma that affects approximately 200 million people worldwide. Schistosomiasis has been a persistent problem in endemic areas as there is no vaccine available, currently used anti-helmintic medications do not prevent reinfection, and most concerning, drug resistance has been documented in laboratory and field isolates. Thus, alternative approaches to curtail this human disease are warranted. Understanding the immunobiology of the obligate intermediate host of these parasites, which include the freshwater snail Biomphalaria glabrata, may facilitate the development of novel methods to stop or reduce transmission to humans. Molecules from the thioester-containing protein (TEP) superfamily have been shown to be involved in immunological functions in many animals including corals and humans. In this study we identified, characterized, and compared TEP transcripts and their expression upon S. mansoni exposure in resistant and susceptible strains of B. glabrata snails. Results showed the expression of 11 unique TEPs in B. glabrata snails. These transcripts present high sequence identity at the nucleotide and putative amino acid levels between susceptible and resistant strains. Further analysis revealed differences in several TEPs’ constitutive expression levels between resistant and susceptible snail strains, with C3-1, C3-3, and CD109 having higher constitutive expression levels in the resistant (BS90) strain, whereas C3-2 and TEP-1 showed higher constitutive expression levels in the susceptible (NMRI) strain. Furthermore, TEP-specific response to S. mansoni miracidia exposure reiterated their differential expression, with resistant snails upregulating the expression of both TEP-4 and TEP-3 at 2 h and 48 h post-exposure, respectively. Further understanding the diverse TEP genes and their functions in invertebrate animal vectors will not only expand our knowledge in regard to this ancient family of immune proteins, but also offer the opportunity to identify novel molecular targets that could aid in the efforts to develop control methods to reduce schistosomiasis transmission.

Scratching the Itch: Updated Perspectives on the Schistosomes Responsible for Swimmer's Itch around the World

Although most studies of digenetic trematodes of the family Schistosomatidae dwell on representatives causing human schistosomiasis, the majority of the 130 identified species of schistosomes infect birds or non-human mammals. The cercariae of many of these species can cause swimmer's itch when they penetrate human skin. Recent years have witnessed a dramatic increase in our understanding of schistosome diversity, now encompassing 17 genera with eight more lineages awaiting description. Collectively, schistosomes exploit 16 families of caenogastropod or heterobranch gastropod intermediate hosts. Basal lineages today are found in marine gastropods and birds, but subsequent diversification has largely taken place in freshwater, with some reversions to marine habitats. It seems increasingly likely that schistosomes have on two separate occasions colonized mammals. Swimmer's itch is a complex zoonotic disease manifested through several different routes of transmission involving a diversity of different host species. Swimmer's itch also exemplifies the value of adopting the One Health perspective in understanding disease transmission and abundance because the schistosomes involved have complex life cycles that interface with numerous species and abiotic components of their aquatic environments. Given the progress made in revealing their diversity and biology, and the wealth of questions posed by itch-causing schistosomes, they provide excellent models for implementation of long-term interdisciplinary studies focused on issues pertinent to disease ecology, the One Health paradigm, and the impacts of climate change, biological invasions and other environmental perturbations.

Host preference of field‐derived Schistosoma mansoni is influenced by snail host compatibility and infection status

Schistosome parasites cause a chronic inflammatory disease in humans, and recent studies have emphasized the importance of control programs for understanding the aquatic phases of schistosomiasis transmission. The host-seeking behavior of larval schistosomes (miracidia) for their snail intermediate hosts plays a critical role in parasite transmission. Using field-derived strains of Kenyan snails and parasites, we tested two main hypotheses: (1) Parasites prefer the most compatible host, and (2) parasites avoid hosts that are already infected. We tested preference to three Biomphalaria host snail taxa (B. pfeifferi, B. sudanica, and B. choanomphala), using allopatric and sympatric Schistosoma mansoni isolates and two different nonhost snail species that co-occur with Biomphalaria, Bulinus globosus, and Physa acuta. We also tested whether schistosomes avoid snail hosts that are already infected by another trematode species and whether competitive dominance played a role in their behavior. Preference was assessed using two-way choice chambers and by visually counting parasites that moved toward competing stimuli. In pairwise comparisons, we found that S. mansoni did not always prefer the more compatible snail taxon, but never favored an incompatible host over a compatible host. While parasites preferred B. pfeifferi to the nonhost species B. globosus, they did not significantly prefer B. pfeifferi versus P. acuta, an introduced species in Kenya. Finally, we demonstrated that parasites avoid infected snails if the resident parasite was competitively dominant (Patagifer sp.), and preferred snails infected with subordinates (xiphidiocercariae) to uninfected snails. These results provide evidence of “fine tuning” in the ability of schistosome miracidia to detect hosts; however, they did not always select hosts that would maximize fitness. Appreciating such discriminatory abilities could lead to a better understanding of how ecosystem host and parasite diversity influences disease transmission and could provide novel control mechanisms to improve human health.

An Overview of Transcriptional Responses of Schistosome-Susceptible (M line) or -Resistant (BS-90) Biomphalaria glabrata Exposed or Not to Schistosoma mansoni Infection

Background

We seek to provide a comprehensive overview of transcriptomics responses of immune-related features of the gastropod Biomphalaria glabrata (Bg) following exposure to Schistosoma mansoni (Sm), a trematode causing human schistosomiasis. Responses of schistosome-susceptible (M line, or SUS) and -resistant (BS-90, or RES) Bg strains were characterized following exposure to Sm for 0.5, 2, 8 or 40 days post-exposure (dpe).

Methods

RNA-Seq and differential expression analysis were undertaken on 56 snails from 14 groups. We considered 7 response categories: 1) constitutive resistance factors; 2) constitutive susceptibility factors; 3) generalized stress responses; 4) induced resistance factors; 5) resistance factors suppressed in SUS snails; 6) suppressed/manipulated factors in SUS snails; and 7) tolerance responses in SUS snails. We also undertook a gene co-expression network analysis. Results from prior studies identifying schistosome resistance/susceptibility factors were examined relative to our findings.

Results

A total of 792 million paired-end reads representing 91.2% of the estimated 31,985 genes in the Bg genome were detected and results for the 7 categories compiled and highlighted. For both RES and SUS snails, a single most supported network of genes with highly correlated expression was found.

Conclusions

1) Several constitutive differences in gene expression between SUS and RES snails were noted, the majority over-represented in RES; 2) There was little indication of a generalized stress response shared by SUS and RES snails at 0.5 or 2 dpe; 3) RES snails mounted a strong, multi-faceted response by 0.5 dpe that carried over to 2 dpe; 4) The most notable SUS responses were at 40 dpe, in snails shedding cercariae, when numerous features were either strongly down-regulated indicative of physiological distress or parasite manipulation, or up-regulated, suggestive of tolerance or survival-promoting effects; 5) Of 55 genes previously identified in genome wide mapping studies, 29 (52.7%) were responsive to Sm, as were many familiar resistance-associated genes (41.0%) identified by other means; 6) Both network analysis and remarkably specific patterns of expression of lectins and G protein-coupled receptors in categories 4, 6 and 7 were indicative of orchestrated responses of different suites of genes in SUS or RES snails following exposure to Sm.

Toll like receptors and their evolution in the lymnaeid freshwater snail species Radix auricularia and Lymnaea stagnalis, key intermediate hosts for zoonotic trematodes

One of the major evolutionarily conserved pathways in innate immunity of invertebrates is the toll-like receptor (TLR) pathway. However, little is known of the TLR protein family in gastropod molluscs despite their role in the transmission of human diseases, especially the common lymnaeid freshwater snail species Radix auricularia and Lymnaea stagnalis, key intermediate hosts of zoonotic trematodes. Using comparative genomics and gene prediction approaches utilising the freshwater snail Biomphalaria glabrata genome as a reference ten putative TLR proteins were identified in both R. auricularia and L. stagnalis. Phylogenetic analyses revealed that unlike other molluscs the lymnaeid species also possessed class 1 TLRs, previously thought to be unique to B. glabrata. Gene duplication events were also seen across the TLR classes in the lymnaeids with several of the genes appearing to exist as potential tandem elements in R. auricularia. Each predicted TLR was shown to possess the typical the leucine-rich repeat extracellular and TIR intracellular domains and both single cysteine clusters and multiple cysteine clusters TLRs were identified in both lymnaeid species. Principle component analyses of 3D models of the predicted TLRs showed that class 1 and 5 proteins did not cluster based on similarity of structure, suggested to be potential adaptation to a range of pathogens. This study provides the first detailed account of TLRs in lymnaeids and affords a platform for further research into the role of these proteins into susceptibility and compatibility of these snails with trematodes and their role in transmission.

Host Controls of Within-Host Disease Dynamics: Insight from an Invertebrate System

AbstractWithin-host processes (representing the entry, establishment, growth, and development of a parasite inside its host) may play a key role in parasite transmission but remain challenging to observe and quantify. We develop a general model for measuring host defenses and within-host disease dynamics. Our stochastic model breaks the infection process down into the stages of parasite exposure, entry, and establishment and provides associated probabilities for a host's ability to resist infections with barriers and clear internal infections. We tested our model on Daphnia dentifera and the parasitic fungus Metschnikowia bicuspidata and found that when faced with identical levels of parasite exposure, Daphnia patent (transmitting) infections depended on the strength of internal clearance. Applying a Gillespie algorithm to the model-estimated probabilities allowed us to visualize within-host dynamics, within which signatures of host defense could be clearly observed. We also found that early within-host stages were the most vulnerable to internal clearance, suggesting that hosts have a limited window during which recovery can occur. Our study demonstrates how pairing longitudinal infection data with a simple model can reveal new insight into within-host dynamics and mechanisms of host defense. Our model and methodological approach may be a powerful tool for exploring these properties in understudied host-parasite interactions.

Mobilizing molluscan models and genomes in biology

Molluscs are among the most ancient, diverse, and important of all animal taxa. Even so, no individual mollusc species has emerged as a broadly applied model system in biology. We here make the case that both perceptual and methodological barriers have played a role in the relative neglect of molluscs as research organisms. We then summarize the current application and potential of molluscs and their genomes to address important questions in animal biology, and the state of the field when it comes to the availability of resources such as genome assemblies, cell lines, and other key elements necessary to mobilising the development of molluscan model systems. We conclude by contending that a cohesive research community that works together to elevate multiple molluscan systems to 'model' status will create new opportunities in addressing basic and applied biological problems, including general features of animal evolution. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

RNA-seq: The early response of the snail Physella acuta to the digenetic trematode Echinostoma paraensei

To analyze the response of the snail Physella acuta to Echinostoma paraensei, a compatible digenetic trematode, Illumina RNA-seq data were collected from snails with early infection (5 snails at 2 days post-exposure [DPE]) and established infection (4 snails, 8 DPE), and 7 control (unexposed) snails. A reference transcriptome (325,563 transcripts, including 98% of eukaryotic universal single-copy orthologs; BUSCO) and a draft P. acuta genome (employing available genomic Illumina reads; 799,945 scaffolds, includes 88% BUSCO genes) were assembled to guide RNA-seq analyses. Parasite exposure of P. acuta led to 10,195 differentially expressed (DE) genes at 2 DPE and 8,876 DE genes at 8 DPE with only 18% of up-regulated and 22% of down-regulated sequences shared between these time points. Gene ontology (GO) analysis yielded functional annotation of only 1.2% of DE genes but did not indicate major changes in biological activities of P. acuta between 2 and 8 DPE. Increased insights were achieved by analysis of expression profiles of 460 immune-relevant DE transcripts, identified by BLAST and InterProScan. Physella acuta has expanded gene families that encode immune-relevant domains, including CD109/TEP, GTPase IMAP, Limulus agglutination factor (dermatopontin), FReD (≥82 sequences with fibrinogen-related domains), and transcripts that combine C-type lectin (C-LECT) and C1q domains, novel among metazoa. Notably, P. acuta expressed sequences from these immune gene families at all time points, but the assemblages of unique transcripts from particular immune gene families differed between 2 and 8 DPE. The shift in profiles of DE immune genes, from early exposure to parasite establishment, suggests that compatible P. acuta initially respond to infection but switch to express immune genes that likely are less effective against E. paraensei but counter other types of (opportunistic) pathogens and parasites. We propose that the latter expression profile is part of an extended phenotype of E. paraensei, imposed upon P. acuta through parasite manipulation of the host, following successful parasite establishment in the snail after 2 DPE.

Genomic and transcriptional analysis of genes containing fibrinogen and IgSF domains in the schistosome vector Biomphalaria glabrata, with emphasis on the differential responses of snails susceptible or resistant to Schistosoma mansoni

Achieving a deeper understanding of the factors controlling the defense responses of invertebrate vectors to the human-infecting pathogens they transmit will provide needed new leads to pursue for control. Consequently, we provide new genomic and transcriptomic insights regarding FReDs (containing a fibrinogen domain) and FREPs (fibrinogen domain and one or two IgSF domains) from the planorbid snail Biomphalaria glabrata, a Neotropical vector of Schistosoma mansoni, causative agent of human intestinal schistosomiasis. Using new bioinformatics approaches to improve annotation applied to both genome and RNA-Seq data, we identify 73 FReD genes, 39 of which are FREPs. We provide details of domain structure and consider relationships and homologies of B. glabrata FBG and IgSF domains. We note that schistosome-resistant (BS-90) snails mount complex FREP responses following exposure to S. mansoni infection whereas schistosome-susceptible (M line) snails do not. We also identify several coding differences between BS-90 and M line snails in three FREPs (2, 3.1 and 3.2) repeatedly implicated in other studies of anti-schistosome responses. In combination with other results, our study provides a strong impetus to pursue particular FREPs (2, 3.1, 3.2 and 4) as candidate resistance factors to be considered more broadly with respect to schistosome control efforts, including involving other Biomphalaria species vectoring S. mansoni in endemic areas in Africa.

Just as malaria parasites depend on mosquitoes for transmission, Schistosoma trematodes that infect 200+ million people in developing countries depend on freshwater snails to support their development. New control methods targeting schistosomes in the snail host are urgently needed to help break the cycle of transmission. Identification of molecules in snails that confer or facilitate resistance to successful larval development offers one way forward. Building on previous studies and new data, we used a combination of bioinformatics tools to characterize a repertoire of molecules in B. glabrata containing fibrinogen or IgSF domains, or both (the latter molecules called FREPs, first discovered in B. glabrata). We found that snails resistant to schistosomes mount vigorous, multi-component FREP responses following exposure to schistosomes whereas responses of susceptible snails are modest by comparison. Furthermore, for four FREPs repeatedly implicated in snail responses to trematodes, three (FREPs 2, 3.1 and 3.2) exist in distinctive allelic forms in susceptible and resistant snails. We propose that the search for homologous molecules needs to move to the field, to B. glabrata in the Neotropics and to other vector species of Biomphalaria in Africa, to explore the potential of FREPs as resistance factors worthy of development as control agents.

Diminished adherence of Biomphalaria glabrata embryonic cell line to sporocysts of Schistosoma mansoni following programmed knockout of the allograft inflammatory factor

BACKGROUND

Larval development in an intermediate host gastropod snail of the genus Biomphalaria is an obligatory component of the life-cycle of Schistosoma mansoni. Understanding of the mechanism(s) of host defense may hasten the development of tools that block transmission of schistosomiasis. The allograft inflammatory factor 1, AIF, which is evolutionarily conserved and expressed in phagocytes, is a marker of macrophage activation in both mammals and invertebrates. AIF enhances cell proliferation and migration. The embryonic cell line, termed Bge, from Biomphalaria glabrata is a versatile resource for investigation of the snail-schistosome relationship since Bge exhibits a hemocyte-like phenotype. Hemocytes perform central roles in innate and cellular immunity in gastropods and in some cases can kill the parasite. However, the Bge cells do not kill the parasite in vitro.

METHODS

Bge cells were transfected by electroporation with plasmid pCas-BgAIFx4, encoding the Cas9 nuclease and a guide RNA specific for exon 4 of the B. glabrata AIF (BgAIF) gene. Transcript levels for Cas9 and for BgAIF were monitored by reverse-transcription-PCR and, in parallel, adhesion of gene-edited Bge cells during co-culture with of schistosome sporocysts was assessed.

RESULTS

Gene knockout manipulation induced gene-disrupting indels, frequently 1-2 bp insertions and/or 8-30 bp deletions, at the programmed target site; a range from 9 to 17% of the copies of the BgAIF gene in the Bge population of cells were mutated. Transcript levels for BgAIF were reduced by up to 73% (49.5 ± 20.2% SD, P ≤ 0.05, n = 12). Adherence by BgAIF gene-edited (ΔBgAIF) Bge to sporocysts diminished in comparison to wild type cells, although cell morphology did not change. Specifically, as scored by a semi-quantitative cell adherence index (CAI), fewer ΔBgAIF than control wild type cells adhered to sporocysts; control CAI, 2.66 ± 0.10, ΔBgAIF, 2.30 ± 0.22 (P ≤ 0.01).

CONCLUSIONS

The findings supported the hypothesis that BgAIF plays a role in the adherence of B. glabrata hemocytes to sporocysts during schistosome infection in vitro. This demonstration of the activity of programmed gene editing will enable functional genomics approaches using CRISPR/Cas9 to investigate additional components of the snail-schistosome host-parasite relationship.

No evidence for a dilution effect of the non-native snail, Potamopyrgus antipodarum, on native snails

The dilution effect can occur by a range of mechanisms and results in reduced parasite prevalence in host taxa. In invaded ecosystems, the dilution effect can benefit native species if non-native species, acting as resistant or less competent hosts, reduce rates of parasitic infections in native species. In field experiments, we assessed whether manipulating biomass of the non-native snail, Potamopyrgus antipodarum, caused a dilution effect by reducing trematode infections in three taxa of native snails. In contrast to many studies showing resistant or less competent non-native hosts can “dilute” or reduce infection rates, we found no evidence for a dilution effect reducing infection rates of any of the native snails. We suggest that a dilution effect may not have occurred because most trematode taxa are highly host specific, and thus the trematode transmission stages did not recognize the invasive snail as a possible host. In this case, community composition appears to be important in influencing the dilution effect.

NEONATAL SUSCEPTIBILITY TO INFECTION WITH SCHISTOSOMA MANSONI IN RESISTANT BIOMPHALARIA GLABRATA

The laboratory BS-90 strain of the freshwater pulmonate snail Biomphalaria glabrata, progeny of snails collected from Salvador, Brazil, is resistant to infection with Schistosoma mansoni as juveniles or adults, which rapidly kill primary sporocysts with an attack by the internal defense system (IDS). However, neonatal snails are susceptible to infection. Although neonatal susceptibility of Salvador B. glabrata was reported in 1953 and confirmed subsequently, this phenomenon has been largely ignored. In this study, susceptibility was examined in discrete sizes (shell diameters) of BS-90 snails. We found that 1 mm snails are highly susceptible and develop patent infections. Unexpectedly, most infected 1 mm snails contain primary sporocysts in the digestive gland. Snails measuring 2 and 3 mm show reduced prevalence of infection, and 4 mm and larger snails are refractory. In snails larger than 1 mm, sporocysts fail to develop normally, as shown by reduced numbers of germinal cells at 48 hr post exposure. Moreover, in larger snails an increasingly stronger response of the IDS is mounted in the form of increased numbers of sporocysts undergoing encapsulation and destruction by hemocytes, increased layers of encapsulating hemocytes, as well as increased mitotic activity of the hematopoietic amebocyte-producing organ. These results indicate a relatively narrow size range over which resistance develops and suggest that the IDS of 1 mm snails is developmentally immature. The occurrence of infections in neonatal snails may help to explain transmission of schistosomiasis in regions of low snail susceptibility and may complicate future efforts in biological control.

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.

Scaphanocephalus-associated dermatitis as the basis for black spot disease in Acanthuridae of St. Kitts, West Indies

Acanthurus spp. of St. Kitts and other Caribbean islands, including ocean surgeonfish A. bahianus, doctorfish A. chirurgus, and blue tang A. coeruleus, frequently show multifocal cutaneous pigmentation. Initial reports from the Leeward Antilles raised suspicion of a parasitic etiology. The aim of this study was to quantify the prevalence of the disease in St. Kitts' Acanthuridae and describe its pathology and etiology. Visual surveys demonstrated consistently high adjusted mean prevalence at 3 shallow reefs in St. Kitts in 2017 (38.9%, 95% CI: 33.8-43.9) and 2018 (51.5%; 95% CI: 46.2-56.9). There were no differences in prevalence across species or reefs, but juvenile fish were less commonly affected than adults. A total of 29 dermatopathy-affected acanthurids were sampled by spearfishing for comprehensive postmortem examination. Digenean metacercariae were dissected from <1 mm cysts within pigmented lesions. Using partial 28S rDNA sequence data they were classified as Family Heterophyidae, members of which are commonly implicated in black spot disease of other fishes. Morphological features of the parasite were most typical of Scaphanocephalus spp. (Creplin, 1842), and 2 genetic profiles were obtained suggesting more than 1 digenean species. Histologically, pigmented lesions had mild chronic perivascular dermatitis and increased melanophores and melanin density, often centered on encysted digenean metacercariae. In 1 affected A. chirurgus, similar metacercariae were histologically identified in skeletal and cardiac muscle. Further research is needed to clarify impact on host fitness, establish the number of heterophyid digenean species that cause black spots on Caribbean fishes and to determine the intermediate and definitive host species.

Comparison of NMRI Schistosoma mansoni Infection in Three Susceptible Laboratory Albino Strains of Biomphalaria glabrata

Albino strains of Biomphalaria glabrata that are compatible with Schistosoma mansoni are commonly used to investigate snail-schistosome interactions, but whether they are all equally compatible is not known. In this study, compatibility with the Naval Medical Research Institute (NMRI) strain of S. mansoni was compared among 3 widely used albino strains: NMRI (the normal laboratory host for NMRI S. mansoni), M line, and University of Massachusetts Lowell (UML). Compatibility was assessed on the basis of infection prevalence following exposure to miracidia, the histological fate of sporocysts, and mitotic response in the snail amebocyte-producing organ (APO), a component of the internal defense system (IDS). Infection prevalence in UML was nearly identical to that in NMRI but was significantly lower in M line. Although the APO of UML showed no response to infection over the course of 9 days, mitotic activity was elevated in the APO of NMRI and M line, with that in M line being higher and more prolonged than in the APO of resistant BS-90 snails. Finally, hemocyte responses against some small primary sporocysts occurred at 1 and 3 days post-exposure (DPE) in all 3 strains, but in 2 of 5 M line a response also occurred against large primary sporocysts at 6 DPE. Thus, based on infection prevalence and tissue responses, compatibility with NMRI S. mansoni is lowest in M line, whereas UML and NMRI show the same degree of compatibility, despite decades of maintenance of this parasite strain in NMRI snails. The elevated mitotic response in the APO of M line and NMRI snails suggests that a response of the IDS can occur even in a compatible host-parasite relationship.

Human protein paucimannosylation: cues from the eukaryotic kingdoms

Paucimannosidic proteins (PMPs) are bioactive glycoproteins carrying truncated α- or β-mannosyl-terminating asparagine (N)-linked glycans widely reported across the eukaryotic domain. Our understanding of human PMPs remains limited, despite findings documenting their existence and association with human disease glycobiology. This review comprehensively surveys the structures, biosynthetic routes and functions of PMPs across the eukaryotic kingdoms with the aim of synthesising an improved understanding on the role of protein paucimannosylation in human health and diseases. Convincing biochemical, glycoanalytical and biological data detail a vast structural heterogeneity and fascinating tissue- and subcellular-specific expression of PMPs within invertebrates and plants, often comprising multi-α1,3/6-fucosylation and β1,2-xylosylation amongst other glycan modifications and non-glycan substitutions e.g. O-methylation. Vertebrates and protists express less-heterogeneous PMPs typically only comprising variable core fucosylation of bi- and trimannosylchitobiose core glycans. In particular, the Manα1,6Manβ1,4GlcNAc(α1,6Fuc)β1,4GlcNAcβAsn glycan (M2F) decorates various human neutrophil proteins reportedly displaying bioactivity and structural integrity demonstrating that they are not degradation products. Less-truncated paucimannosidic glycans (e.g. M3F) are characteristic glycosylation features of proteins expressed by human cancer and stem cells. Concertedly, these observations suggest the involvement of human PMPs in processes related to innate immunity, tumorigenesis and cellular differentiation. The absence of human PMPs in diverse bodily fluids studied under many (patho)physiological conditions suggests extravascular residence and points to localised functions of PMPs in peripheral tissues. Absence of PMPs in Fungi indicates that paucimannosylation is common, but not universally conserved, in eukaryotes. Relative to human PMPs, the expression of PMPs in plants, invertebrates and protists is more tissue-wide and constitutive yet, similar to their human counterparts, PMP expression remains regulated by the physiology of the producing organism and PMPs evidently serve essential functions in development, cell-cell communication and host-pathogen/symbiont interactions. In most PMP-producing organisms, including humans, the N-acetyl-β-hexosaminidase isoenzymes and linkage-specific α-mannosidases are glycoside hydrolases critical for generating PMPs via N-acetylglucosaminyltransferase I (GnT-I)-dependent and GnT-I-independent truncation pathways. However, the identity and structure of many species-specific PMPs in eukaryotes, their biosynthetic routes, strong tissue- and development-specific expression, and diverse functions are still elusive. Deep exploration of these PMP features involving, for example, the characterisation of endogenous PMP-recognising lectins across a variety of healthy and N-acetyl-β-hexosaminidase-deficient human tissue types and identification of microbial adhesins reactive to human PMPs, are amongst the many tasks required for enhanced insight into the glycobiology of human PMPs. In conclusion, the literature supports the notion that PMPs are significant, yet still heavily under-studied biomolecules in human glycobiology that serve essential functions and create structural heterogeneity not dissimilar to other human N-glycoprotein types. Human PMPs should therefore be recognised as bioactive glycoproteins that are distinctly different from the canonical N-glycoprotein classes and which warrant a more dedicated focus in glycobiological research.

Derivatives of the lectin complement pathway in Lophotrochozoa

A plethora of non-overlapping immune molecular mechanisms in metazoans is the most puzzling issue in comparative immunobiology. No valid evolutionary retrospective on these mechanisms has been developed. In this study, we aimed to reveal the origin and evolution of the immune complement-like system in Lophotrochozoa. For this, we analyzed publicly available transcriptomes of prebilaterian and lophotrochozoan species, mapping lineage-specific molecular events on the phylogenetic tree. We found that there were no orthologs of mannose-binding lectin (MBL) and ficolins (FCN) in Lophotrochozoa but C1q-like proteins (C1qL), bearing both a collagen domain and a globular C1q domain, were omnipresent in them. This suggests that among all complement-like activators the C1qL-specific domain architecture was an evolutionarily first. Two novel protostomian MASP-Related Molecules, MReM1 and MReM2, might hypothetically compensate for the loss of a prebilaterian MASP-orthologous gene and act in complex with C1qL and C1qDC as a "proto-activator" of an ancient "proto-complement". We proposed a new model of the complement evolution predicting that numerous lineage-specific complement-like systems should have evolved from a stem "antique" molecular complex. First evolved in the common ancestor of coelomic animals, the "antique" humoral complex consisted of a TEP molecule, the common ancestor of TEP-associated proteases (C2/Bf/Сf/Lf), the common ancestor of MASP-like proteases (MASP/C1r/C1s, MReM1/MReM2) and multimeric recognition proteins (C1q-, MBL- and FCN-homologs). Further evolutionary specialization and expansion of the complex was independent and lineage-specific, examples being the mammalian complement system and the Apogastropoda complement-like complex. The latter includes an impressive array of multimeric recognition proteins, the variable immunoglobulin and lectin domain containing molecules (VIgL), homologous to C1q, MBL, FCN and other lectins. Four novel polymorphic subfamilies of VIgLs were found to be expressed in Apogastropoda: C1q-related proteins (QREP), zona pellucida-related proteins (ZREP), Scavenger Receptor Cys-Rich-related proteins (SREP) and HPA-lectin related proteins (HREP). The transcriptional response of fibrinogen-related proteins of VIgL family (LlFREP), LlQREP and LlSREP to infestation of common periwinkle, Littorina littorea, with digenean parasite Himasthla elongata correlates with that of LlMReM1, supporting the model suggested in this study.

Morphological characterization and molecular phylogeny of zoonotic trematodes in the freshwater snail Asolene platae

In the context of a broader program dealing with the symbiotic associations of apple snails, we sampled three species that coexist in Lake Regatas (Palermo, Buenos Aires, Argentina). The population of Asolene platae, (but not those of Pomacea canaliculata and Pomacea scalaris) showed trematode larvae (echinocercariae and xiphidiocercariae) in haemocoelic spaces and connective tissue of the digestive gland. The echinocercariae resembled those of the genus Echinochasmus, but lacked sensory hairs on their body and tail; whereas xiphidiocercariae were similar to the xiphidiocercariae armatae belonging to the Opisthoglyphe type. The phylogenetical positions of these trematodes were inferred by the 28S rRNA, ITS1 and mtCOXI gene sequences. The 28S rRNA gene linked the echinocercarial sequences with the polyphyletic genus Echinochasmus (Echinochasmidae), while the xiphidiocercarial sequences were linked with the genus Phaneropsolus (Phaneropsolidae). The molecular markers used were able to distinguish two cryptic molecular entities of the single echinocercarial morphotype. Although ITS1 and mtCOXI did not allow resolving phylogeny beyond the family level because of the scarce number of sequences in the molecular databases, both cercariae (echinocercariae and xiphidiocercariae) could be distinguished by the Internal Transcribed Spacer 1 amplicon size. This is the first correlative morphological and molecular study of zoonotic trematodes in Neotropical ampullariids.

Proteomic Analysis of Biomphalaria glabrata Hemocytes During in vitro Encapsulation of Schistosoma mansoni Sporocysts

Circulating hemocytes of the snail Biomphalaria glabrata, a major intermediate host for the blood fluke Schistosoma mansoni, represent the primary immune effector cells comprising the host's internal defense system. Within hours of miracidial entry into resistant B. glabrata strains, hemocytes infiltrate around developing sporocysts forming multi-layered cellular capsules that results in larval death, typically within 24–48 h post-infection. Using an in vitro model of hemocyte-sporocyst encapsulation that recapitulates in vivo events, we conducted a comparative proteomic analysis on the responses of hemocytes from inbred B. glabrata strains during the encapsulation of S. mansoni primary sporocysts. This was accomplished by a combination of Laser-capture microdissection (LCM) to isolate sections of hemocyte capsules both in the presence and absence of sporocysts, in conjunction with mass spectrometric analyses to establish protein expression profiles. Comparison of susceptible NMRI snail hemocytes in the presence and absence of sporocysts revealed a dramatic downregulation of proteins in during larval encapsulation, especially those involved in protein/CHO metabolism, immune-related, redox and signaling pathways. One of 4 upregulated proteins was arginase, competitor of nitric oxide synthetase and inhibitor of larval-killing NO production. By contrast, when compared to control capsules, sporocyst-encapsulating hemocytes of resistant BS-90 B. glabrata exhibited a more balanced profile with enhanced expression of shared proteins involved in protein synthesis/processing, immunity, and redox, and unique expression of anti-microbial/anti-parasite proteins. A final comparison of NMRI and BS-90 host hemocyte responses to co-cultured sporocysts demonstrated a decrease or downregulation of 77% of shared proteins by NMRI cells during encapsulation compared to those of the BS-90 strain, including lipopolysaccharide-binding protein, thioredoxin reductase 1 and hemoglobins 1 and 2. Overall, using this in vitro model, results of our proteomic analyses demonstrate striking differences in proteins expressed by susceptible NMRI and resistant BS-90 snail hemocytes to S. mansoni sporocysts during active encapsulation, with NMRI hemocytes exhibiting extensive downregulation of protein expression and a lower level of constitutively expressed immune-relevant proteins (e.g., FREP2) compared to BS-90. Our data suggest that snail strain differences in hemocyte protein expression during the encapsulation process account for observed differences in their cytotoxic capacity to interact with and kill sporocysts.

The putative immune recognition repertoire of the model cnidarian Hydractinia symbiolongicarpus is large and diverse

Immune recognition of molecular patterns from microorganisms or self-altered cells activate effector responses that neutralize and eliminate these potentially harmful agents. In virtually every metazoan group the process is carried out by pattern recognition receptors, typically constituted by immunoglobulin (Ig), leucine rich repeat (LRR), and/or lectin domains. In order to get insights into the ancestral immune recognition repertoire of animals, we have sequenced the transcriptome of bacterially challenged colonies of the model cnidarian Hydractinia symbiolongicarpus using the Illumina platform. Over 116,000 assembled contigs were annotated by sequence similarity, domain architecture, and functionally. From these, a subset of 315 unique transcripts was predicted as the putative immune recognition repertoire of H. symbiolongicarpus. Interestingly, canonical Toll-like receptors (TLR) were not predicted, nor any transmembrane protein with the Toll/interleukine-1 receptor (TIR) domain. Yet, a variety of predicted proteins with transmembrane domains associated with LRR ectodomains were identified, as well as homologs of the key transduction factor NF-kB, and its associated regulatory proteins. This also has been documented in Hydra, and suggests that recognition and signaling initiation has been decoupled in the TLR system of hydrozoans. In contrast, both canonical and non-canonical NOD-like receptors were identified in H. symbiolongicarpus, showing a higher diversity than the TLR system and perhaps a wider functional landscape. The collection of Ig-like containing putative immune recognition molecules was diverse, and included at least 26 unique membrane-bound predicted proteins and 88 cytoplasmic/secreted predicted molecules. In addition, 25 and 5 transcripts encoding the Ig-like containing allorecognition determinants ALR1 and ALR2, respectively, were identified. Sequence and phylogenetic analyses suggested the presence of various transcriptionally active alr loci, and the action of recombination-based mechanisms diversifying them. Transcripts encoding at least six lectin families with putative roles in immune recognition were found, including 19 unique C-type lectins and 21 unique rhamnose-binding lectins. Other predicted immune recognition receptors included scavenger receptors from three families, lipopolysaccharide-binding proteins, cell-adhesion molecules and thioester-bond containing proteins. This analysis demonstrated that the putative immune recognition repertoire of H. symbiolongicarpus is large and diverse.

Identification and functional characterization of thioredoxin-related protein of 14 kDa in Oncomelania hupensis, the intermediate host of Schistosoma japonicum

Oncomelania hupensis is the unique intermediate host of the blood fluke Schistosoma japonicum, which causes schistosomiasis. In snails, highly toxic reactive oxygen species (ROS) can be continually generated by hemocytes in response to foreign particles or pathogens, and may be involved in damaging and eliminating digenean larvae. Thioredoxin-related protein of 14 kDa (TRP14) is a member of the Trx superfamily, and plays an important role in the scavenging of ROS. This study was designed to identify and characterize TRP14 from O. hupensis (OhTRP14), and investigate the involvement of OhTRP14 in the scavenging of ROS in snail host immune response to the parasite S. japonicum. Here we expressed and purified the recombinant OhTRP14 and its mutant, and rOhTRP14 displayed oxidoreductase activity dependent on the CPDC motif. OhTRP14 protein was ubiquitously present in all the tested snail tissues, and especially immunolocalized in the cytoplasm of immune cell types (hemocytes). Both the expression of OhTRP14 and ROS level increased significantly in snails following challenge with S. japonicum. The dsRNA-mediated knockdown of OhTRP14 was successfully conducted by oral feeding, and ROS production was increased by OhTRP14 knockdown, implying that OhTRP14 was involved in the scavenging of ROS in O. hupensis circulating hemocytes. Therefore, we conclude that OhTRP14 may be involved in the scavenging of ROS in snail host immune response to the parasite S. japonicum. The results expand our understanding of the interaction between this parasite and host, and lay a foundation for the establishment of Oncomelania-schistosome infection models.

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.

Reversal of Schistosome Resistance In Biomphalaria glabrata By Heat Shock May Be Dependent On Snail Genotype

Several genes have recently been shown to affect the innate resistance of laboratory strains of Biomphalaria glabrata to infection with Schistosoma mansoni, including Hsp90, the expression of which following a brief exposure to elevated temperature can interfere with resistance in juvenile BS-90 snails. Because a prior study failed to see a similar effect in adult snails, juvenile BS-90 snails were exposed to 10 or 50 miracidia following a 4.5 to 6 hr incubation at 33 or 37 C. Snails were then monitored for production of secondary sporocysts or release of cercariae. In addition, snails exposed to 33 C were examined histologically between 1 and 15 days post exposure (DPE) to 30 miracidia to assess the fate of primary sporocysts. Other than elevated numbers of viable primary sporocysts in the tissues of heat shocked snails at 3 DPE, no statistically significant effect of elevated temperature was observed. This discrepancy with regard to prior studies is hypothesized to result from genetic divergence in different laboratory colonies of the same strain of snail.

Identification of Toll-like receptor family members in Oncomelania hupensis and their role in defense against Schistosoma japonicum

The amphibious snail, Oncomelania hupensis, primarily distributed in the Far East, is the only intermediate host of Schistosoma japonicum, which causes the most virulent form of schistosomiasis. Obligatory parasitism of snails is the main vehicle for human and livestock infection and depends primarily on parasite infectivity, snail defense capacity and specificity, and parasite-snail compatibility. Therefore, the schistosome-snail interaction is biomedically significant, particularly the molecular mechanisms involved in the innate immune response against S. japonicum. Several immune effectors and signaling pathways have been successfully identified in mollusks, especially in Biomphalaria glabrata, the intermediate snail host of S. mansoni; however, limited information is available for O. hupensis. Here, we identified 16 Toll-like receptors (TLRs) in O. hupensis. These O. hupensis TLRs (OhTLRs) are highly expressed in haemocytes, the primary immune cell of mollusks. Most of the OhTLRs were more highly expressed in female gonads than in other tissues, which may suggest maternal immune transfer in O. hupensis. After S. japonicum challenge, the expression levels of all of the OhTLRs were significantly up-regulated at 6 h post-challenge; many of the OhTLR expression levels were inhibited at later time points in haemocytes, while they were inhibited and fluctuated to varying degrees in other tissues. Additionally, we further determined the tissue-specific expression and dynamic response against S. japonicum of one of the TLR signaling adaptors, myeloid differentiation factor 88 (MyD88), from O. hupensis. Three OhMyD88 genes were highly expressed in haemocytes, and were up-regulated in haemocytes and inhibited in the head-foot muscle at the early time-point after S. japonicum challenge; however, these had slower changes and longer durations compared to OhTLRs. These results provide evidence suggesting that immune effectors are involved in innate immune responses of O. hupensis against S. japonicum and may play a role in the activation of different haemocytes, and not limited for the early response to S. japonicum invasion. Further investigation into the varied expression of OhTLRs in other tissues after S. japonicum challenge will improve our understanding of TLR function in innate immunity of O. hupensis.

Novel approach to study gastropod-mediated innate immune reactions against metastrongyloid parasites

The anthropozoonotic metastrongyloid nematodes Angiostrongylus cantonensis and Angiostrongylus costaricensis, as well as Angiostrongylus vasorum, Crenosoma vulpis, Aelurostrongylus abstrusus and Troglostrongylus brevior are currently considered as emerging gastropod-borne parasites and have gained growing scientific attention in the last years. However, the knowledge on invertebrate immune responses and on how metastrongyloid larvae are attacked by gastropod immune cells is still limited. This work aims to describe an in vitro system to investigate haemocyte-derived innate immune responses of terrestrial gastropods induced by vital axenic metastrongyloid larvae. We also provide protocols on slug/snail management and breeding under standardized climate conditions (circadian cycle, temperature and humidity) for the generation of parasite-free F0 stages which are essential for immune-related investigations. Adult slug species (Arion lusitanicus, Limax maximus) and giant snails (Achatina fulica) were maintained in fully automated climate chambers until mating and production of fertilized eggs. Newly hatched F0 juvenile specimens were kept under parasite-free conditions before experimental use. An improved protocol for gastropod haemolymph collection and haemocyte isolation was established. Giemsa-stained haemolymph preparations showed adequate haemocyte isolation in all three gastropod species. Additionally, a protocol for the production of axenic first and third stage larvae (L1, L3) was established. Haemocyte functionality was tested in haemocyte-nematode-co-cultures. Scanning electron microscopy (SEM) and light microscopy analyses revealed that gastropod-derived haemocytes formed clusters as well as DNA-rich extracellular aggregates catching larvae and decreasing their motility. These data confirm the usefulness of the presented methods to study haemocyte-mediated gastropod immune responses to better understand the complex biology of gastropod-borne diseases.

Integrated multi-omic analyses in Biomphalaria-Schistosoma dialogue reveal the immunobiological significance of FREP-SmPoMuc interaction

The fresh water snail Biomphalaria glabrata is one of the vectors of the trematode pathogen Schistosoma mansoni, which is one of the agents responsible of human schistosomiasis. In this host-parasite interaction, co-evolutionary dynamic results into an infectivity mosaic known as compatibility polymorphism. Integrative approaches including large scale molecular approaches have been conducted in recent years to improve our understanding of the mechanisms underlying compatibility. This review presents the combination of integrated Multi-Omic approaches leading to the discovery of two repertoires of polymorphic and/or diversified interacting molecules: the parasite antigens S. mansoni polymorphic mucins (SmPoMucs) and the B. glabrata immune receptors fibrinogen-related proteins (FREPs). We argue that their interactions may be major components for defining the compatible/incompatible status of a specific snail/schistosome combination.

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.

Genomic evidence for population-specific responses to co-evolving parasites in a New Zealand freshwater snail

Reciprocal co-evolving interactions between hosts and parasites are a primary source of strong selection that can promote rapid and often population- or genotype-specific evolutionary change. These host-parasite interactions are also a major source of disease. Despite their importance, very little is known about the genomic basis of co-evolving host-parasite interactions in natural populations, especially in animals. Here, we use gene expression and sequence evolution approaches to take critical steps towards characterizing the genomic basis of interactions between the freshwater snail Potamopyrgus antipodarum and its co-evolving sterilizing trematode parasite, Microphallus sp., a textbook example of natural coevolution. We found that Microphallus-infected P. antipodarum exhibit systematic downregulation of genes relative to uninfected P. antipodarum. The specific genes involved in parasite response differ markedly across lakes, consistent with a scenario where population-level co-evolution is leading to population-specific host-parasite interactions and evolutionary trajectories. We also used an F_ST -based approach to identify a set of loci that represent promising candidates for targets of parasite-mediated selection across lakes as well as within each lake population. These results constitute the first genomic evidence for population-specific responses to co-evolving infection in the P. antipodarum-Microphallus interaction and provide new insights into the genomic basis of co-evolutionary interactions in nature.

Towards interruption of schistosomiasis transmission in sub-Saharan Africa: developing an appropriate environmental surveillance framework to guide and to support 'end game' interventions

Schistosomiasis is a waterborne parasitic disease in sub-Saharan Africa, particularly common in rural populations living in impoverished conditions. With the scale-up of preventive chemotherapy, national campaigns will transition from morbidity- to transmission-focused interventions thus formal investigation of actual or expected declines in environmental transmission is needed as 'end game' scenarios arise. Surprisingly, there are no international or national guidelines to do so in sub-Saharan Africa. Our article therefore provides an introduction to key practicalities and pitfalls in the development of an appropriate environmental surveillance framework. In this context, we discuss how strategies need to be adapted and tailored to the local level to better guide and support future interventions through this transition. As detection of egg-patent infection in people becomes rare, careful sampling of schistosome larvae in freshwater and in aquatic snails with robust species-specific DNA assays will be required. Appropriate metrics, derived from observed prevalence(s) as compared with predetermined thresholds, could each provide a clearer insight into contamination- and exposure-related dynamics. Application could be twofold, first to certify areas currently free from schistosomiasis transmission or second to red-flag recalcitrant locations where extra effort or alternative interventions are needed.

Study of surface carbohydrates in Galba truncatula tissues before and after infection with Fasciola hepatica

The presence and distribution of surface carbohydrates in the tissues of Galba truncatula snails uninfected or after infection with Fasciola hepatica as well as on the surface of the snail-pathogenic larval stages of the parasite were studied by lectin labelling assay. This is an attempt to find similarities that indicate possible mimicry, utilised by the parasite as an evasion strategy in this snail-trematode system. Different binding patterns were identified on head-foot-mantle, hepatopancreas, genital glands, renopericardial complex of the host as well as of the snail-pathogenic larval stages of F. hepatica. The infection with F. hepatica leads to changes of labelling with Glycine max in the head-mantle cells and Arachis hypogaea in the tubular epithelium of the hepatopancreas. The lectin binding on the other snail tissues is not changed by the development of the larvae. Our data clearly demonstrated the similarity in labelling of G. truncatula tissues and the surface of the snail-pathogenic larval stages of F. hepatica. The role of glycosylation of the contact surfaces of both organisms in relation to the host-parasite interactions is also discussed.

Relative compatibility of Schistosoma mansoni with Biomphalaria sudanica and B. pfeifferi from Kenya as assessed by PCR amplification of the S. mansoni ND5 gene in conjunction with traditional methods

Background

Schistosoma mansoni is hosted by several species of Biomphalaria spp. snails in Africa. We were interested in determining if there were differences in compatibility of S. mansoni with Biomphalaria sudanica from Lake Victoria, or with B. pfeifferi from streams and smaller water bodies in Kenya. Does this parasite develop with equal efficiency in both snail species, and does this have implications for transmission in different habitat types?

Methods

Primers for PCR amplification of the S. mansoni ND5 gene were designed and tested for sensitivity and specificity. We exposed laboratory-reared B. sudanica and field-derived B. pfeifferi to single miracidium infections and at 1, 2, 4, 8, 16 and 24 days post-exposure (dpe), snails were extracted for the PCR assay. Snails were also shed for cercariae and/or dissected prior to extraction. Additionally, B. sudanica and B. pfeifferi were collected from field locations and tested with the PCR assay.

Results

The ND5 PCR assay was sensitive (>0.1 fg S. mansoni genomic DNA) and allowed S. mansoni to be differentiated from other relevant schistosome species or snails. The number of PCR positive snails at 1–4 dpe was higher for B. pfeifferi than for B. sudanica, but not significantly so (P = 0.052). From 8–24 dpe, more B. pfeifferi harbored successfully developing parasites (positive by both dissection and PCR) than did B. sudanica (P = 0.008). At 40 dpe, more B. pfeifferi than B. sudanica shed cercariae or harbored dissection positive/PCR positive infections (P < 0.001). Both immature and failed (dissection negative but PCR positive) S. mansoni infections could also be detected in naturally infected snails of both species.

Conclusions

The PCR assay detected S. mansoni infections in snails exposed to one miracidium for one day. Both B. sudanica and B. pfeifferi supported full development of S. mansoni, but B. pfeifferi was more compatible, with significantly more dissection positive/PCR positive or shedding infections, and significantly fewer failed infections (dissection negative/PCR positive). This highlights the relatively lower compatibility of B. sudanica with S. mansoni, and suggests the factors responsible for incompatibility and how they might affect transmission of S. mansoni in habitats like Lake Victoria deserve additional study.

Pathogen-associated molecular patterns activate expression of genes involved in cell proliferation, immunity and detoxification in the amebocyte-producing organ of the snail Biomphalaria glabrata

The anterior pericardial wall of the snail Biomphalaria glabrata has been identified as a site of hemocyte production, hence has been named the amebocyte-producing organ (APO). A number of studies have shown that exogenous abiotic and biotic substances, including pathogen associated molecular patterns (PAMPs), are able to stimulate APO mitotic activity and/or enlarge its size, implying a role for the APO in innate immunity. The molecular mechanisms underlying such responses have not yet been explored, in part due to the difficulty in obtaining sufficient APO tissue for gene expression studies. By using a modified RNA extraction technique and microarray technology, we investigated transcriptomic responses of APOs dissected from snails at 24 h post-injection with two bacterial PAMPs, lipopolysaccharide (LPS) and peptidoglycan (PGN), or with fucoidan (FCN), which may mimic fucosyl-rich glycan PAMPs on sporocysts of Schistosoma mansoni. Based upon the number of genes differentially expressed, LPS exhibited the strongest activity, relative to saline-injected controls. A concurrent activation of genes involved in cell proliferation, immune response and detoxification metabolism was observed. A gene encoding checkpoint 1 kinase, a key regulator of mitosis, was highly expressed after stimulation by LPS. Also, seven different aminoacyl-tRNA synthetases that play an essential role in protein synthesis were found to be highly expressed. In addition to stimulating genes involved in cell proliferation, the injected substances, especially LPS, also induced expression of a number of immune-related genes including arginase, peptidoglycan recognition protein short form, tumor necrosis factor receptor, ficolin, calmodulin, bacterial permeability increasing proteins and E3 ubiquitin-protein ligase. Importantly, significant up-regulation was observed in four GiMAP (GTPase of immunity-associated protein) genes, a result which provides the first evidence suggesting an immune role of GiMAP in protostome animals. Moreover, altered expression of genes encoding cytochrome P450, glutathione-S-transferase, multiple drug resistance protein as well as a large number of genes encoding enzymes associated with degradation and detoxification metabolism was elicited in response to the injected substances.

Gastropod-Borne Helminths: A Look at the Snail-Parasite Interplay

More than 300 million people suffer from a range of diseases caused by gastropod-borne helminths, predominantly flatworms and roundworms, whose life cycles are characterized by a diversified ecology and epidemiology. Despite the plethora of data on these parasites, very little is known of the fundamental biology of their gastropod intermediate hosts, or of the interactions occurring at the snail-helminth interface. In this article, we focus on schistosomes and metastrongylids of human and animal significance, and review current knowledge of snail-parasite interplay. Future efforts aimed at elucidating key elements of the biology and ecology of the snail intermediate hosts, together with an improved understanding of snail-parasite interactions, will aid to identify, plan, and develop new strategies for disease control focused on gastropod intermediate hosts.

Multistep Fractionation and Mass Spectrometry Reveal Zwitterionic and Anionic Modifications of the N- and O-glycans of a Marine Snail

Various studies in the past have revealed that molluscs can produce a wide range of rather complex N-glycan structures, which vary from those occurring in other invertebrate animals; particularly methylated glycans have been found in gastropods, and there are some reports of anionic glycans in bivalves. Due to the high variability in terms of previously described structures and methodologies, it is a major challenge to establish glycomic workflows that yield the maximum amount of detailed structural information from relatively low quantities of sample. In this study, we apply differential release with peptide:N-glycosidases F and A followed by solid-phase extraction on graphitized carbon and reversed-phase materials to examine the glycome of Volvarina rubella (C. B. Adams, 1845), a margin snail of the clade Neogastropoda. The resulting four pools of N-glycans were fractionated on a fused core RP-HPLC column and subject to MALDI-TOF MS and MS/MS in conjunction with chemical and enzymatic treatments. In addition, selected N-glycan fractions, as well as O-glycans released by β-elimination, were analyzed by porous graphitized carbon-LC-MS and MS(n). This comprehensive approach enabled us to determine a number of novel modifications of protein-linked glycans, including N-methyl-2-aminoethylphosphonate on mannose and N-acetylhexosamine residues, core β1,3-linked mannose, zwitterionic moieties on core Galβ1,4Fuc motifs, additional mannose residues on oligomannosidic glycans, and bisubstituted antennal fucose; furthermore, typical invertebrate N-glycans with sulfate and core fucose residues are present in this gastropod.

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.

Praziquantel efficacy against Brachylaima sp. metacercariae (Trematoda: Brachylaimidae) parasitizing the edible landsnail Cornu aspersum and its HPLC-MS/MS residue determination

Cornu aspersum is a terrestrial edible snail, often parasitized by Brachylaima (Trematoda) metacercariae. Ingestion of undercooked snails by humans allows metacercariae to develop to adult in the intestine causing brachylaimiasis (expected mortality rate 5-10%). The cosmopolitan character of the trematode and of its vector, enhanced in a future climate change scenario, and the absence of adequate sanitary controls of the snails in marketplaces clearly increase the risk of human brachylaimiasis. The treatment of farmed snails with praziquatel (PZQ) would be a tool to control this food-borne disease. The objectives of this study were: to report the prevalence of Brachylaima metacercariae parasitizing C. aspersum in marketplaces, to assess the efficacy and tolerance of PZQ in C. aspersum and to develop an HPLC-MS/MS analytical method to quantify PZQ residue in the edible snail (the acceptable daily intake of PZQ is 0.17 mg/kg bw in humans). Prevalence of parasitization of C. aspersum by Brachylaima metacercariae in public marketplaces reached the 80%. PZQ was orally administered to snails, mixed with the usual snail food. In dose determination assays three doses were individually tested (10 days): 1.2 mg PZQ/snail, 1.8 mg PZQ/snail (efficacy 97.3%, p < 0.001) and 2.4 mg PZQ/snail (efficacy 98.7%, p < 0.001). In dose confirmation tests (n = 200) the 2.4 mg PZQ/snail dose was group tested (10 snails/box, 7 days): 2 g of PZQ supplemented snail food were disposed daily in the treatment group boxes and 2 g of snail food (placebo) in the control boxes (efficacy 94.6%, p < 0.001; prevalence dropped from 68.7% in control group to 10.1% in treatment group, p < 0.001). Received dose was 220.2 mg PZQ/kg snail with shell. In the analytical method, linearity, lower limit of quantification (0.05 μg/ml), selectivity, carry over, accuracy, precision, dilution integrity, matrix effect and stability were tested. Sixty snails were treated (11 mg PZQ/g snail food) and analyzed. PZQ was detected and quantified (0.093 mg PZQ/g snail). PZQ treatment of C. aspersum is effective, well tolerated by the snail, affordable and easy to reproduce in snail farms.

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.