Biochemical Characterization of Oyster and Clam Galectins: Selective Recognition of Carbohydrate Ligands on Host Hemocytes and Perkinsus Parasites

Differences in phagosome acidification of Manila clam hemocytes in response to two Perkinsus species with contrasting proliferation dynamics in the host: P. olseni and P. mediterraneus

Understanding the factors determining the host ranges of Perkinsus spp., a significant group of pathogenic protozoans affecting shellfish, is essential for preventing their spread and designing effective control measures. Considering that differences in the ability to proliferate within the host may influence the determination of host range, we first injected six Perkinsus spp. into Manila clams Ruditapes philippinarum and monitored the variations of trophozoite numbers. Although all six species were detected in the challenged clams 28 days post infection, the infection intensities varied among species, and particularly two species showed contrasting infection trends: P. mediterraneus showed a decreasing trend of infection, declining to the lowest intensity, whereas that of P. olseni continuously increased, reaching the highest intensity. In vitro exposure to Manila clam hemocytes revealed that the survival of P. mediterraneus trophozoites was suppressed, in contrast to P. olseni, which maintained their viability. Despite similar phagocytic indices for both species, the rate of phagosome acidification was significantly higher for hemocytes phagocytizing P. mediterraneus than those targeting P. olseni. Notably, phagosome acidification was significantly suppressed in hemocytes phagocytizing live P. olseni trophozoites, suggesting that P. olseni may secrete a substance that modulates phagosome acidification, and thereby evades intracellular digestion by the host's hemocytes. Conversely, P. mediterraneus, with a lower affinity for infecting Manila clams, did not exhibit such modulation. Based on these results, we consider that the ability to modulate phagosome acidification in host hemocytes might be at least one factor in determining the host range of Perkinsus species.

Harnessing the power of mollusc lectins as immuno-protective biomolecules

The rapid advancement of molecular research on macromolecules has contributed to the discovery of 'Lectin', a carbohydrate-binding protein which specifically interacts with receptors on the surface of glycans and regulates various cellular activities thereby stimulating immunological functions. Considering the wide variety of sources and immunological significance, research has led to the discovery of lectins in invertebrate molluscs. Such lectins in molluscs mediate active immune response as they lack adaptive immunity. Phylum Mollusca is identified with different types of lectins such as C-lectin, Galectin, P-lectin, I-lectin, and H-lectin, along with other immunologically significant lectin molecules such as F- lectin, R-lectin, ficolins, chitinase like lectin etc., all of these with specific ligand binding and structural diversity. Molluscan C-type lectins are the most functional ones that increase the activity of phagocytic cells through specific carbohydrate binding of antigenic ligands and haemocyte adhesion thereby enhancing the immune response. Helix pomatia agglutinin and Helix aspersa agglutinin are the two H-lectins that were identified within molluscs that could even target cancer-progressing cells through specific binding. Also, these lectins identified in molluscs are proven to be efficient in antibacterial and immunomodulatory functions. These insights attract researchers to identify novel lectins in molluscs and their characterization that play a key role in protection against diseases. This review discusses the structural features of mollusc lectins, their specific binding, molecular interactions and their immunological applications.

The mechanisms involved in byssogenesis in Pteria penguin under different temperatures

Byssus is important for marine bivalves to adhere robustly to diverse substrates and resist environmental impacts. The winged pearl oyster, Pteria penguin, can reattach or not reattach to the same environment, which leaves the development and survival of the oyster population at risk. In this study, diverse methods were employed to evaluate the byssus quality and explore the mechanism of byssus secretion at different temperatures. The results demonstrated that oysters maintained their byssus properties at different temperatures through polyphenol oxidase (PPO) and reactive oxygen species (ROS) variation. They were both higher at 27 °C than at 21 °C. Furthermore, PPO activities of WB27 (31.78 U/g ± 1.50 U/g) were significantly higher than NB27, WB21, and NB21. Sectional observation revealed three types of vesicles, from which a novel vesicle might participate in byssogenesis as a putative metal storage particle. Moreover, cytoskeletal proteins may cooperate with cilia to transport byssal proteins, which then facilitate byssus formation under the regulation of upstream signals. Transcriptome analysis demonstrated that protein quality control, ubiquitin-mediated proteolysis, and cytoskeletal reorganization-related genes contributed to adaptation to temperature changes and byssus fabrication, and protection-related genes play a critical role in byssogenesis, byssus toughness, and durability. These results were utilized to create a byssogenesis mechanism model, to reveal the foot gland and vesicle types of P. penguin and provide new insights into adaptation to temperature changes and byssus fabrication in sessile bivalves.

A brief history of galectin evolution

Galectins are a family of carbohydrate-binding proteins found in vertebrates in great abundance and diversity in terms of both structure and ligand-binding properties as well as physiological function. Proteins with clear relationships to vertebrate galectins are already found in primitive Bilateria. The increasing amount of accessible well-annotated bilaterian genomes has allowed us to reveal, through synteny analyses, a new hypothesis about the phylogenetic history of the galectin family in this animal group. Thus, we can trace the genomic localization of the putative ancestral Bilateria galectin back to the scallops as a still very primitive slow-evolving bilaterian lineage. Intriguingly, our analyses show that the primordial galectin of the Deuterostomata most likely exhibited galectin-8-like characteristics. This basal standing galectin is characterized by a tandem-repeat type with two carbohydrate recognition domains as well as by a sialic acid binding property of the N-terminal domain, which is typical for galectin-8. With the help of synteny, the amplification of this potential primordial galectin to the broad galectin cosmos of modern jawed vertebrates can be reconstructed. Therefore, it is possible to distinguish between the paralogs resulting from small-scale duplication and the ohnologues generated by whole-genome duplication. Our findings support a substantially new hypothesis about the origin of the various members of the galectin family in vertebrates. This allows us to reveal new theories on the kinship relationships of the galectins of Gnatostomata. In addition, we focus for the first time on the galectines of the Cyclostomata, which as a sister group of jawed vertebrates providing important insights into the evolutionary history of the entire subphylum. Our studies also highlight a previously neglected member of the galectin family, galectin-related protein 2. This protein appears to be a widespread ohnologue of the original tandem-repeat ancestor within Gnathostomata that has not been the focus of galectin research due to its nonclassical galactose binding sequence motif and the fact that it was lost during mammalian evolution.

N-glycan breakdown by bacterial CAZymes

The modification of proteins by N-glycans is ubiquitous to most organisms and they have multiple biological functions, including protecting the adjoining protein from degradation and facilitating communication or adhesion between cells, for example. Microbes have evolved CAZymes to deconstruct different types of N-glycans and some of these have been characterised from microbes originating from different niches, both commensals and pathogens. The specificity of these CAZymes provides clues as to how different microbes breakdown these substrates and possibly cross-feed them. Discovery of CAZymes highly specific for N-glycans also provides new tools and options for modifying glycoproteins.

Bivalve Haemocyte Subpopulations: A Review

Bivalve molluscs stand out for their ecological success and their key role in the functioning of aquatic ecosystems, while also constituting a very valuable commercial resource. Both ecological success and production of bivalves depend on their effective immune defence function, in which haemocytes play a central role acting as both the undertaker of the cellular immunity and supplier of the humoral immunity. Bivalves have different types of haemocytes, which perform different functions. Hence, identification of cell subpopulations and their functional characterisation in immune responses is essential to fully understand the immune system in bivalves. Nowadays, there is not a unified nomenclature that applies to all bivalves. Characterisation of bivalve haemocyte subpopulations is often combined with 1) other multiple parameter assays to determine differences between cell types in immune-related physiological activities, such as phagocytosis, oxidative stress and apoptosis; and 2) immune response to different stressors such as pathogens, temperature, acidification and pollution. This review summarises the major and most recent findings in classification and functional characterisation of the main haemocyte types of bivalve molluscs.

Perkinsus marinus suppresses in vitro eastern oyster apoptosis via IAP-dependent and caspase-independent pathways involving TNFR, NF-kB, and oxidative pathway crosstalk

The protozoan parasite Perkinsus marinus causes Dermo disease in eastern oysters, Crassostrea virginica, and can suppress apoptosis of infected hemocytes using incompletely understood mechanisms. This study challenged hemocytes in vitro with P. marinus for 1 h in the presence or absence of caspase inhibitor Z-VAD-FMK or Inhibitor of Apoptosis protein (IAP) inhibitor GDC-0152. Hemocytes exposure to P. marinus significantly reduced granulocyte apoptosis, and pre-incubation with Z-VAD-FMK did not affect P. marinus-induced apoptosis suppression. Hemocyte pre-incubation with GDC-0152 prior to P. marinus challenge further reduced apoptosis of granulocytes with engulfed parasite, but not mitochondrial permeabilization. This suggests P. marinus-induced apoptosis suppression may be caspase-independent, affect an IAP-involved pathway, and occur downstream of mitochondrial permeabilization. P. marinus challenge stimulated hemocyte differential expression of oxidation-reduction, TNFR, and NF-kB pathways. WGCNA analysis of P. marinus expression in response to hemocyte exposure revealed correlated protease, kinase, and hydrolase expression that could contribute to P. marinus-induced apoptosis suppression.

Genomics and Transcriptomics of the green mussel explain the durability of its byssus

Mussels, which occupy important positions in marine ecosystems, attach tightly to underwater substrates using a proteinaceous holdfast known as the byssus, which is tough, durable, and resistant to enzymatic degradation. Although various byssal proteins have been identified, the mechanisms by which it achieves such durability are unknown. Here we report comprehensive identification of genes involved in byssus formation through whole-genome and foot-specific transcriptomic analyses of the green mussel, Perna viridis. Interestingly, proteins encoded by highly expressed genes include proteinase inhibitors and defense proteins, including lysozyme and lectins, in addition to structural proteins and protein modification enzymes that probably catalyze polymerization and insolubilization. This assemblage of structural and protective molecules constitutes a multi-pronged strategy to render the byssus highly resistant to environmental insults.

Galectin-mediated immune recognition: Opsonic roles with contrasting outcomes in selected shrimp and bivalve mollusk species

Galectins are a structurally conserved family of ß-galactoside-binding lectins characterized by a unique sequence motif in the carbohydrate recognition domain, and of wide taxonomic distribution, from fungi to mammals. Their biological functions, initially described as key to embryogenesis and early development via recognition of endogenous ("self") carbohydrate moieties, are currently understood as also encompassing tissue repair, cancer metastasis, angiogenesis, adipogenesis, and regulation of immune homeostasis. More recently, however, numerous studies have contributed to establish a new paradigm by revealing that galectins can also bind to exogenous ("non-self") glycans on the surface of potentially pathogenic virus, bacteria, and eukaryotic parasites, and function both as pathogen recognition receptors (PRRs) and effector factors in innate immunity. Our studies on a galectin from the kuruma shrimp Marsupenaeus japonicus (MjGal), revealed that it functions as a typical PRR. Expression of MjGal is upregulated by infectious challenge, and can recognize both Gram (+) and Gram (-) bacteria. MjGal also recognizes carbohydrates on the shrimp hemocyte surface, and can cross-link microbial pathogens to the hemocytes, promoting their phagocytosis and clearance from circulation. Therefore, MjGal contributes to the shrimp's immune defense against infectious challenge both as a PRR and effector factor. Our studies on galectins from the bivalve mollusks, however, have shown that although they can function in immune defense as MjGal, protistan parasites take advantage of the recognition roles of the host galectins, for successful attachment and host infection. We identified in the eastern oyster Crassostrea virginica two galectins (CvGal1 and CvGal2) that not only recognize a large variety of bacterial species, but also the protozoan parasite Perkinsus marinus. Like the shrimp MjGal, both oyster galectins function as opsonins, and promote parasite adhesion and phagocytosis. However, P. marinus survives intrahemocytic oxidative killing and proliferates, eventually causing systemic infection and death of the oyster host. In the softshell clam Mya arenaria we identified a galectin (MaGal1) that displays carbohydrate specificity and recognition properties for sympatric Perkinsus species (P. marinus and P. chesapeaki), that are different from CvGal1 and CvGal2. Our results suggest that although galectins from bivalves can function as PRRs, Perkinsus parasites have co-evolved with their hosts to subvert the galectins' immune functions for host infection by acquisition of carbohydrate-based mimicry.

Genome to phenome tools: In vivo and in vitro transfection of Crassostrea virginica hemocytes

The sequencing of the Crassostrea virginica genome has brought back the interest for gene delivery and editing methodologies. Here, we report the expression in oyster hemocytes of two heterologous expression vectors under the CMV promoter delivered with dendrimers. Expression was monitored using confocal microscopy, flow cytometry, and immunofluorescence assay. C. virginica hemocytes were able to express the green fluorescence protein and Crassostrea gigas vascular endothelial growth factor under CMV viral promoter both in vivo and in vitro. These results provide the bases for interrogating the genome and adapting genome editing methodologies.

A qPCR-Based Survey of Haplosporidium nelsoni and Perkinsus spp. in the Eastern Oyster, Crassostrea virginica in Maine, USA

Eastern oyster (Crassostrea virginica) aquaculture is increasingly playing a significant role in the state of Maine's (USA) coastal economy. Here, we conducted a qPCR-based survey for Haplosporidium nelsoni, Perkinsus marinus, and Perkinsus chesapeaki in C. virginica (n = 1440) from six Maine sites during the summer-fall of 2016 and 2017. In the absence of reported die-offs, our results indicated the continued presence of the three protozoan parasites in the six sites. The highest H. nelsoni qPCR-prevalence corresponded to Jack's Point and Prentiss Island (x=40 and 48% respectively), both located in the Damariscotta River Estuary. Jack's Point, Prentiss Island, New Meadows River, and Weskeag River recorded the highest qPCR-prevalence for P. marinus (32-39%). While the P. marinus qPCR-prevalence differed slightly for the years 2016 and 2017, P. chesapeaki qPCR-prevalence in 2016 was markedly lower than 2017 (<20% at all sites versus >60% at all sites for each of the years, respectively). Mean qPCR-prevalence values for P. chesapeaki over the two-year study were ≥40% for samples from Jack's Point (49%), Prentiss Island (44%), and New Meadows River (40%). This study highlights that large and sustained surveys for parasitic diseases are fundamental for decision making toward the management of the shellfish aquaculture industry, especially for having a baseline in the case that die-offs occur.