Effect of a toxicant on phagocytosis pathways in the freshwater snail Lymnaea stagnalis

Effects of a Sudden Drop in Salinity on Immune Response Mechanisms of Anadara kagoshimensis

In this experiment, the effects of a sudden drop of salinity on the immune response mechanisms of the ark shell Anadara kagoshimensis were examined by simulating the sudden drop of salinity that occurs in seawater after a rainstorm. Additionally, the differentially expressed genes (DEGs) were identified using transcriptome sequencing. When the salinity dropped from 30‱ (S30) to 14‱ (S14), the phagocytic activity of blood lymphocytes, the O₂^- levels produced from respiratory burst, the content of reactive oxygen species, and the activities of lysozymes and acid phosphatases increased significantly, whereas the total count of blood lymphocytes did not increase. Total count of blood lymphocytes in 22‱ salinity (S22) was significantly higher than that in any other group. The raw data obtained from sequencing were processed with Trimmomatic (Version 0.36). The expression levels of unigenes were calculated using transcripts per million (TPM) based on the effects of sequencing depth, gene length, and sample on reads. Differential expression analysis was performed using DESeq (Version 1.12.4). Transcriptome sequencing revealed 269 (101 up-regulated, 168 down-regulated), 326 (246 up-regulated, 80 down-regulated), and 185 (132 up-regulated, 53 down-regulated) significant DEGs from comparison of the S14 vs. S22, S22 vs. S30, and S14 vs. S30 groups, respectively. Gene Ontology enrichment analysis of the DEGs in these salinity comparison groups revealed that the cellular amino acid metabolic process, the regulation of protein processing, the regulation of response to stress, and other terms were significantly enriched. Kyoto Encyclopedia of Genes and Genomes enrichment analysis showed that nucleotide-binding, oligomerization domain (NOD)-like receptor signaling pathway (ko04621), apoptosis-multiple species (ko04215), Toll and Imd signaling pathway (ko04624), NF-κB signaling pathway (ko04064), apoptosis (ko04210), and focal adhesion (ko04510) were significantly enriched in all salinity comparison groups. qRT-PCR verification of 12 DEGs in the above six pathways was conducted, and the results were consistent with the transcriptome sequencing results in terms of up-regulation and down-regulation, which illustrates that the transcriptome sequencing data are credible. These results were used to preliminarily explore the effects of a sudden drop of salinity on blood physiological and biochemical indexes and immunoregulatory mechanisms of A. kagoshimensis. They also provide a theoretical basis for the selection of bottom areas optimal for release and proliferation of A. kagoshimensis required to restore the declining populations of this species.

The hematopoiesis in gill and its role in the immune response of Pacific oyster Crassostrea gigas against secondary challenge with Vibrio splendidus

Increasing evidences have demonstrated that the invertebrate gill is a predominant tissue participating in the immune response during pathogen challenge. In the present study, the hematopoiesis and immune activities in gill of Pacific oyster Crassostrea gigas were investigated. Stem-like cells with big nuclei and thin cytoplasm were found in the tubules of gill filaments, where DNA synthesis is active and hemocytes production are exuberant. The oysters primarily stimulated by formaldehyde-killed Vibrio splendidus exhibited stronger immune responses and enhanced cell regeneration in gill when they encountered the secondary challenge of live V. splendidus. After the secondary stimulation with V. splendidus, the expression levels of CgClec-4 and CgIFN in the gill of oysters pre-stimulated with formaldehyde-killed V. splendidus were significantly higher (p < 0.05) than that in the oysters pre-stimulated with filter-sterilized (0.22 μm pore size) sea water, while the expression level of CgIL-17 was significantly decreased (p < 0.05). Meanwhile, the protein expression level of hematopoietic transcription factor CgGATA3 and immune-related protein CgEcSOD in gill increased apparently after the secondary challenge with V. splendidus. ROS production was also enhanced (p < 0.05) at 6 h and 24 h after the secondary challenge. The phagocytic rate in gill of oysters pre-stimulated with formaldehyde-killed V. splendidus was significantly increased (p < 0.05) at 6 h after the secondary challenge with live V. splendidus, showing faster response than that pre-stimulated with filter-sterilized sea water. These results collectively showed that the immune parameters in gill were apparently enhanced after secondary challenge with live V. splendidus, indicating that hematopoiesis might participate in immune priming in Pacific oyster C. gigas.

Analysis of hemocytes in Lymnaea stagnalis: Characterization and effects of repeated hemolymph collections

The first part of the study was devoted to test the hypothesis according to which the hemolymph of Lymnaea stagnalis can be collected repeatedly - regardless the time-intervals - at an individual scale without impact on survival nor immunocapacity defined as the hemocyte density and viability. No significant effects on snail survival were observed when repeated hemolymph samplings were performed at frequencies ranging from 96 h up to 24 h. The frequency of hemolymph sampling had no significant effects on hemocyte density but the hemocyte viability was slightly increased for the 24 h frequency group. Hence, we recommend setting the frequency lower than 48 h after two consecutive samplings for further assessment of hemocyte density and viability. Furthermore, a slight "day" effect was observed on snail immunocapacity. These results support the idea that L. stagnalis is a promising gastropod model in environmental immunotoxicology. A time-course analysis of individual hemocytes parameters can be evaluated with a relative confidence in the non-detrimental effect of the sampling. Linear mixed-effect models allow taking the "day" effect into account and so the possible effect of an environmental factor (i.e. xenobiotic exposures) can be analyzed. Statistical inferences indicated that the inter-individual variability for these hemocyte endpoints were on the same order of magnitude than intra-individual variability. The second part of the study was devoted to provide greater insights into the structure/ultrastructure of hemocytes in L. stagnalis. Only one type of hemocyte has been observed. The hemocytes in their free-floating status showed ovoid or spherical shapes. Some hemocytes exerted filopodia and structures shaped like sailboats. Their ultrastructure showed signs of intense cellular activity. Two peculiar organelles were observed. One corresponds to a massive perinuclear structure of dense aspect. The other corresponds to a structure with fibrillary arrangements. These two structures deserve further investigation in order to understand their nature, function and importance in the snails' immunocompetence.

A cytokine-like factor astakine accelerates the hemocyte production in Pacific oyster Crassostrea gigas

Astakine has been reported to be a hematopoietic growth factor of prokineticin homolog firstly found in arthropods freshwater crayfish Pacifastacus leniusculus. In the present study, an astakine homologous gene was identified from Pacific oyster Crassostrea gigas (designated CgAstakine). The full length cDNA of CgAstakine encoded a polypeptide of 103 amino acids containing a prokineticin (PK) domain homologous to that in astakine from freshwater crayfish P. leniusculus. The deduced amino acid sequence of CgAstakine shared higher similarity with those of other invertebrate astakines than prokineticins from vertebrates. The mRNA of CgAstakine was highly expressed in hepatopancreas and adductor muscle of oyster, while the CgAstakine protein was mainly distributed in hepatopancreas, gill and hemocytes. The mRNA expression of CgAstakine in hemocytes was significantly increased (p < 0.01) and maintained at a high level from 3 h to 9 h after Vibrio anguillarum challenge. After the oyster hemocytes were incubated with 5 μg/mL recombinant CgAstakine protein (rCgAstakine) for 24 h in vitro, the proliferation of hemocytes was significantly increased to 1.89 fold of that in control group (p < 0.05). Moreover, the total count of oyster hemocytes was significantly upregulated (2.45 fold of that in control group, p < 0.05) at 12 h after the oysters were received an injection of rCgAstakine (0.5 μg/g). These results collectively indicated that CgAstakine could modulate the hemocytes proliferation both in vitro and in vivo, and probably involved in the hematopoietic process fighting against the invasion of foreign pathogens.

Evidence for inflammation-mediated memory dysfunction in gastropods: putative PLA2 and COX inhibitors abolish long-term memory failure induced by systemic immune challenges

BACKGROUND

Previous studies associate lipid peroxidation with long-term memory (LTM) failure in a gastropod model (Lymnaea stagnalis) of associative learning and memory. This process involves activation of Phospholipase A2 (PLA2), an enzyme mediating the release of fatty acids such as arachidonic acid that form the precursor for a variety of pro-inflammatory lipid metabolites. This study investigated the effect of biologically realistic challenges of L. stagnalis host defense response system on LTM function and potential involvement of PLA2, COX and LOX therein.

RESULTS

Systemic immune challenges by means of β-glucan laminarin injections induced elevated H2O2 release from L. stagnalis circulatory immune cells within 3 hrs of treatment. This effect dissipated within 24 hrs after treatment. Laminarin exposure has no direct effect on neuronal activity. Laminarin injections disrupted LTM formation if training followed within 1 hr after injection but had no behavioural impact if training started 24 hrs after treatment. Intermediate term memory was not affected by laminarin injection. Chemosensory and motor functions underpinning the feeding response involved in this learning model were not affected by laminarin injection. Laminarin's suppression of LTM induction was reversed by treatment with aristolochic acid, a PLA2 inhibitor, or indomethacin, a putative COX inhibitor, but not by treatment with nordihydro-guaiaretic acid, a putative LOX inhibitor.

CONCLUSIONS

A systemic immune challenge administered shortly before behavioural training impairs associative LTM function in our model that can be countered with putative inhibitors of PLA2 and COX, but not LOX. As such, this study establishes a mechanistic link between the state of activity of this gastropod's innate immune system and higher order nervous system function. Our findings underwrite the rapidly expanding view of neuroinflammatory processes as a fundamental, evolutionary conserved cause of cognitive and other nervous system disorders.

In vitro effects of noradrenaline on Akoya pearl oyster (Pinctada imbricata) haemocytes

Exposure to fluctuating environmental conditions in bivalve molluscs can lead to physiological stress and up-regulated production of stress-associated hormones, such as noradenaline (NA). Since environmental stressors have been found to have an immunosuppressive effect on Pinctada imbricata, we investigated the in vitro affects of NA exposure on their defensive haemocytes, focussing specifically on markers of apoptosis. Terminal dUTP nick-end (TUNEL) labelling was used to detect cells displaying DNA fragmentation within tissue exposed to NA. DNA fragmentation was most significant when haemocytes were exposed to 10.0 ng NA/μg protein relative to non-treated controls. Similarly, Annexin V-FITC staining, a marker of early apoptotic events, was evident in cells exposed to 5.0 and 10.0 ng NA/μg protein after 120 min (p<0.05), and haemocyte adhesion to glass slides declined significantly when cells were exposed to 10.0 ng NA/μg protein (p<0.05). A number of morphological and ultrastructural changes in NA-exposed haemocytes were also identified using transmission and scanning electron microscopy. These alterations included chromatin and cytoplasmic condensation, the formation of apoptotic bodies, vacuolisation and blebbing. In NA-treated cells, polymerisation of F-actin was observed around the periphery of the cytoplasm. All of these data suggest that NA induces apoptosis in P. imbricata haemocytes.