Himasthla elongata: effect of infection on expression of the LUSTR-like receptor mRNA in common periwinkle haemocytes

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

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

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

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

Lectin-like molecules in transcriptome of Littorina littorea hemocytes

The common periwinkle Littorina littorea was introduced in the list of models for comparative immunobiology as a representative of phylogenetically important taxon Caenogastropoda. Using Illumina sequencing technology, we de novo assembled the transcriptome of Littorina littorea hemocytes from 182 million mRNA-Seq pair-end 100 bp reads into a total of 15,526 contigs clustered in 4472 unigenes. The transcriptome profile was analyzed for presence of carbohydrate-binding molecules in a variety of architectural contexts. Hemocytes' repertoire of lectin-like proteins bearing conserved carbohydrate-recognition domains (CRDs) is highly diversified, including 11 of 15 lectin families earlier described in animals, as well as the novel members of lectin family found for the first time in mollusc species. The new molluscan lineage-specific domain combinations were confirmed by cloning and sequencing, including the fuco-lectin related molecules (FLReMs) composed of N-terminal region with no sequence homology to any known protein, a middle Fucolectin Tachylectin-4 Pentaxrin (FTP) domain, and a C-terminal epidermal growth factor (EGF) repeat region. The repertoire of lectin-like molecules is discussed in terms of their potential participation in the receptor phase of immune response. In total, immune-associated functions may be attributed to 70 transcripts belonging to 6 lectin families. These lectin-like genes show low overlap between species of invertebrates, suggesting relatively rapid evolution of immune-associated genes in the group. The repertoire provides valuable candidates for further characterization of the gene functions in mollusc immunity.

GPR107, a G-protein-coupled receptor essential for intoxication by Pseudomonas aeruginosa exotoxin A, localizes to the Golgi and is cleaved by furin

A number of toxins, including exotoxin A (PE) of Pseudomonas aeruginosa, kill cells by inhibiting protein synthesis. PE kills by ADP-ribosylation of the translation elongation factor 2, but many of the host factors required for entry, membrane translocation, and intracellular transport remain to be elucidated. A genome-wide genetic screen in human KBM7 cells was performed to uncover host factors used by PE, several of which were confirmed by CRISPR/Cas9-gene editing in a different cell type. Several proteins not previously implicated in the PE intoxication pathway were identified, including GPR107, an orphan G-protein-coupled receptor. GPR107 localizes to the trans-Golgi network and is essential for retrograde transport. It is cleaved by the endoprotease furin, and a disulfide bond connects the two cleaved fragments. Compromising this association affects the function of GPR107. The N-terminal region of GPR107 is critical for its biological function. GPR107 might be one of the long-sought receptors that associates with G-proteins to regulate intracellular vesicular transport.

Himasthla elongata: Implantation of rediae to the specific iteroparous long-living host, Littorina littorea, results in the immune rejection

All semelparous short-lived gastropods studied so far for the experimental transplantation of trematode parthenitae, from one specific host to another, showed more or less successful acceptance of implanted parasites. We implanted echinostomatid rediae, Himasthla elongata, to the specific iteroparous long-living host, coenogastropod Littorina littorea. Using simple and low-invasive implantation techniques we have tested 680 snails injected with 75 redia microhemipopulations (MHP) harvested from naturally infected snails. Neither young nor mature rediae were able to survive in the recipient periwinkles in the course of 30 days post-implantation. A strong immune response from the host was already evident within the first week after implantation: initial inactivation of motile rediae with toxic humoral immune factors, following encapsulation of the implants and increased hemocyte counts. In contrast, rediae from the same MHPs showed perfect survival rates in primary in vitro axenic cultures. The failure of the transplantation experiments is explained in terms of the compatibility matching phenotype model. In the studied host-parasite combination all periwinkles are potentially susceptible and all rediae MHPs are potentially infective, however the probability of the compatible phenotypes matching is virtually low. Low investment in L. littorea annual reproduction would result in increased investment in self maintenance and immune mechanisms, causing the general resistance to the trematode infestation. Presumably, this resistance is relatively higher in long-lived iteroparous gastropods compare to semelparous short-lived mollusks such as pulmonates.

Evidence for an interaction of neuronostatin with the orphan G protein-coupled receptor, GPR107

Neuronostatin, derived from the somatostatin preprohormone, is a recently described peptide that is produced by several tissues involved in cardiovascular regulation and metabolism, including the hypothalamus. Injection of neuronostatin into the lateral cerebroventricle led to a dose-related increase in mean arterial pressure (MAP) in rats. Any attempt to inhibit the production of neuronostatin would alter somatostatin production as well, making determination of the physiological relevance of the peptide's pharmacologic effects by compromise of production approaches impossible. Therefore, we employed an alternative approach to identify and compromise the production of the neuronostatin receptor. Because neuronostatin was shown to signal via a PKA-dependent mechanism, we hypothesized that the neuronostatin receptor was a G protein-coupled receptor (GPCR), in particular, one of the orphan GPCRs for which the ligand is unknown. Therefore, we screened neuronostatin-responsive tissues, including hypothalamus, heart, pancreatic α-cells, and the gastric tumor cell line KATOIII, for expression of orphan GPCRs. Four orphan GPCRs were expressed by all cell types, including GPR56 and GPR107. Knockdown of GPR107, but not GPR56 or GPR146, led to a loss of responsiveness to neuronostatin by KATOIII cells. Rats injected with siRNA directed against GPR107 (2 μg/day for 2 days) into the lateral cerebroventricle did not exhibit an increase in MAP in response to neuronostatin treatment. Rats with compromised GPR107 expression also displayed blunted reactivity in a baroreflex sensitivity test, indicating that GPR107 and neuronostatin may be important regulators of cardiovascular function. Thus, GPR107 is a promising candidate receptor for neuronostatin, and neuronostatin, interacting with GPR107, may play an important role in the central control of cardiovascular function.