Developments in marine invertebrate primary culture reveal novel cell morphologies in the model bivalve Crassostrea gigas

Mechanisms of HAHV-1 Interaction with Hemocytes in Haliotis diversicolor supertexta: An In Vitro Study

Haliotid herpesvirus 1 (HAHV-1) causes significant damage to the abalone aquaculture industry. Knowledge of HAHV-1 invasion and host defense mechanisms is limited due to the lack of stable molluscan cell lines. The present study established an in vitro infection model of HAHV-1 using the primary suspension cultures of hemocytes from Haliotis diversicolor supertexta and Haliotis discus hannai. The cytopathic effects of HAHV-1 on adherent-cultured hemocytes of both species were also investigated. The HAHV-1 DNA loads were firstly monitored by means of quantitative PCR during the development of viral infection, and subsequently the mechanism of interaction between HAHV-1 and hemocytes was explored by means of a transcriptome analysis. H. diversicolor supertexta hemocytes exhibited a high degree of susceptibility to HAHV-1, with viral loads reaching a peak of 4.0 × 10⁷ copies/ng DNA. In contrast, no significant replication was observed in H. discus hannai hemocytes. Transcriptome analysis revealed that HAHV-1 evades the host immune response in the early stages of infection, and hijacks the host's energy and redox metabolism to promote its replication at the late stages. Consequently, this study provides a valuable reference point for the investigation of virus-host interaction between HAHV-1 and abalone in vitro.

Ex Vivo Propagation of Pinctada Birnavirus Using Mantle Tissue Fragment Culture: Application for Measuring Replication at Different Temperatures, TCID50 Assay, and UV Sensitivity

Pinctada birnavirus (PiBV) is the causative agent of summer atrophy in pearl oyster (Pinctada fucata (Gould)). The disease, which induces mass mortality in juveniles less than 1 year old and abnormalities in adults, was first reported in Japan in 2019. Research on the disease has been hindered by the lack of cell lines capable of propagating PiBV. We established an ex vivo method for PiBV propagation using mantle tissue, the primary infection site of the virus. The method was used to investigate the proliferation characteristics of the virus at different culture temperatures and the sensitivity of the virus to UV radiation. The marginal zone of the mantle was found to be the most suitable for PiBV replication in terms of both viral yield and reproducibility. PiBV showed optimal propagation at an incubation temperature of 25 °C, with minimal to no increase at 15 °C or 32.5 °C. Using the tissue culture infectious dose 50 (TCID50) measurement system developed in this study, we found that PiBV propagation was no longer detectable after UV irradiation at 6150 J/m2 or higher. The tissue fragment culture method developed in this study is expected to facilitate both ex vivo experiments and PiBV research.

Exploring the Unknown: How Can We Improve Single-cell RNAseq Cell Type Annotations in Non-model Organisms?

Single-cell RNA sequencing (scRNAseq) is a powerful tool to describe cell types in multicellular organisms across the animal kingdom. In standard scRNAseq analysis pipelines, clusters of cells with similar transcriptional signatures are given cell type labels based on marker genes that infer specialized known characteristics. Since these analyses are designed for model organisms, such as humans and mice, problems arise when attempting to label cell types of distantly related, non-model species that have unique or divergent cell types. Consequently, this leads to limited discovery of novel species-specific cell types and potential mis-annotation of cell types in non-model species while using scRNAseq. To address this problem, we discuss recently published approaches that help annotate scRNAseq clusters for any non-model organism. We first suggest that annotating with an evolutionary context of cell lineages will aid in the discovery of novel cell types and provide a marker-free approach to compare cell types across distantly related species. Secondly, machine learning has greatly improved bioinformatic analyses, so we highlight some open-source programs that use reference-free approaches to annotate cell clusters. Lastly, we propose the use of unannotated genes as potential cell markers for non-model organisms, as many do not have fully annotated genomes and these data are often disregarded. Improving single-cell annotations will aid the discovery of novel cell types and enhance our understanding of non-model organisms at a cellular level. By unifying approaches to annotate cell types in non-model organisms, we can increase the confidence of cell annotation label transfer and the flexibility to discover novel cell types.

Laboratory Replication of Ostreid Herpes Virus (OsHV-1) Using Pacific Oyster Tissue Explants

Pacific oysters (Crassostrea or Magallana gigas) are one of the most economically important aquaculture species globally. Over the past two decades, ostreid herpesvirus (OsHV-1) has become a major pathogen of cultured Pacific oysters, resulting in widespread mortality with a global distribution. Experimental use of OsHV-1 is challenging for many reasons, including both complexity of host-pathogen dynamics and a lack of functioning model systems. The goal of this study was to improve the tools available for working with OsHV-1 in both whole animals and in tissue explants established from oysters maintained in controlled laboratory conditions. Tissue explants were taken from oysters originating from two different sources that have different levels of mortality in experimental OsHV-1 infections and were exposed to OsHV-1. A whole-animal infection experiment was run concurrently as a comparison. Quantitative PCR and electron microscopy were used to confirm that the explants were capable of replicating OsHV-1. Furthermore, the quantitative PCR results suggest that the source of the oysters was significant in determining the outcome of infection in the explants, supporting the validity of the explant model for OsHV-1 infection. This tissue explant approach for studying OsHV-1 allows for the control of confounding factors in the disease outcome that is not possible in whole-animal experiments, providing a new tool for the study of OsHV-1 in Pacific oysters.

Cell-cultivated aquatic food products: emerging production systems for seafood

The demand for fish protein continues to increase and currently accounts for 17% of total animal protein consumption by humans. About 90% of marine fish stocks are fished at or above maximum sustainable levels, with aquaculture propagating as one of the fastest growing food sectors to address some of this demand. Cell-cultivated seafood production is an alternative approach to produce nutritionally-complete seafood products to meet the growing demand. This cellular aquaculture approach offers a sustainable, climate resilient and ethical biotechnological approach as an alternative to conventional fishing and fish farming. Additional benefits include reduced antibiotic use and the absence of mercury. Cell-cultivated seafood also provides options for the fortification of fish meat with healthier compositions, such as omega-3 fatty acids and other beneficial nutrients through scaffold, media or cell approaches. This review addresses the biomaterials, production processes, tissue engineering approaches, processing, quality, safety, regulatory, and social aspects of cell-cultivated seafood, encompassing where we are today, as well as the road ahead. The goal is to provide a roadmap for the science and technology required to bring cellular aquaculture forward as a mainstream food source.

Transcriptomic and Physiological Analysis Reveal Melanin Synthesis-Related Genes and Pathways in Pacific Oysters (Crassostrea gigas)

Shell color is one of the shell traits of molluscs, which has been regarded as an economic trait in some bivalves. Pacific oysters (Crassostrea gigas) are important aquaculture shellfish worldwide. In the past decade, several shell color strains of C. gigas were developed through selective breeding, which provides valuable materials for research on the inheritance pattern and regulation mechanisms of shell color. The inheritance patterns of different shell colors in C. gigas have been identified in certain research; however, the regulation mechanism of oyster pigmentation and shell color formation remains unclear. In this study, we performed transcriptomic and physiological analyses using black and white shell oysters to investigate the molecular mechanism of melanin synthesis in C. gigas. Several pigmentation-related pathways, such as cytochrome P450, melanogenesis, tyrosine metabolism, and the cAMP signaling pathway were found. The majority of differentially expressed genes and some signaling molecules from these pathways exhibited a higher level in the black shell oysters than in the white, especially after L-tyrosine feeding, suggesting that those differences may cause a variation of tyrosine metabolism and melanin synthesis. In addition, the in vitro assay using primary cells from mantle tissue showed that L-tyrosine incubation increased cAMP level, gene and protein expression, and melanin content. This study reveals the difference in tyrosine metabolism and melanin synthesis in black and white shell oysters and provides evidence for the potential regulatory mechanism of shell color in oysters.

An optimized protocol for routine development of cell culture from adult oyster, Crassostrea madrasensis

Marine molluscan cell lines, required for virus screening and cultivation, form essential tools for developing health management strategies for these animals in the blue economy. Moreover, they are also crucial to develop cultivated seafood. As there is no valid marine molluscan cell line, primary cell cultures are relied upon for all investigations. A sound protocol for generating primary cell cultures from molluscs is entailed, but existing protocols often involve heavy antibiotic usage and depuration that invariably affect gene expression and cell health. This work presents an easy-to-adopt, time-saving protocol using non-depurated mollusc Crassostrea madrasensis, which requires only initial antibiotic treatment and minimal exposure or no use of antibiotics in the cell culture medium. The important experimental considerations for arriving at this protocol have been elucidated. Accordingly, sodium hypochlorite and neomycin sulfate were chosen for disinfecting tissues. The study is the first to use shrimp cell culture medium (SCCM) as a cell culture medium for molluscan cell culture. Despite being osmoconformers, the oysters exhibited stable intracellular osmotic conditions and pH, which, when provided in vitro, promoted effective cardiomyocyte formation. The cell viability could be enhanced using 10% fetal bovine serum (FBS), but healthy cell culture could also be obtained using SCCM without FBS. The optimized culture conditions allowed for regular beating cardiomyocyte clusters that could be retained for a month. Limited cell proliferation, as shown by the BrdU assay, demands further interventions, such as possibly producing induced pluripotent stem cells. The optimized protocol and culture conditions also align with some requirements for producing cultivated meat from marine molluscs.

A novel in vivo system to study coral biomineralization in the starlet sea anemone, Nematostella vectensis

Coral conservation requires a mechanistic understanding of how environmental stresses disrupt biomineralization, but progress has been slow, primarily because corals are not easily amenable to laboratory research. Here, we highlight how the starlet sea anemone, Nematostella vectensis, can serve as a model to interrogate the cellular mechanisms of coral biomineralization. We have developed transgenic constructs using biomineralizing genes that can be injected into Nematostella zygotes and designed such that translated proteins may be purified for physicochemical characterization. Using fluorescent tags, we confirm the ectopic expression of the coral biomineralizing protein, SpCARP1, in Nematostella. We demonstrate via calcein staining that SpCARP1 concentrates calcium ions in Nematostella, likely initiating the formation of mineral precursors, consistent with its suspected role in corals. These results lay a fundamental groundwork for establishing Nematostella as an in vivo system to explore the evolutionary and cellular mechanisms of coral biomineralization, improve coral conservation efforts, and even develop novel biomaterials.

Marine diseases and the Anthropocene: Understanding microbial pathogenesis in a rapidly changing world

Healthy marine ecosystems are paramount for Earth's biodiversity and are key to sustaining the global economy and human health. The effects of anthropogenic activity represent a pervasive threat to the productivity of marine ecosystems, with intensifying environmental stressors such as climate change and pollution driving the occurrence and severity of microbial diseases that can devastate marine ecosystems and jeopardise food security. Despite the potentially catastrophic outcomes of marine diseases, our understanding of host-pathogen interactions remains an understudied aspect of both microbiology and environmental research, especially when compared to the depth of information available for human and agricultural systems. Here, we identify three avenues of research in which we can advance our understanding of marine disease in the context of global change, and make positive steps towards safeguarding marine communities for future generations.

Redox control of antioxidants, metabolism, immunity, and development at the core of stress adaptation of the oyster Crassostrea gigas to the dynamic intertidal environment

This review uses the marine bivalve Crassostrea gigas to highlight redox reactions and control systems in species living in dynamic intertidal environments. Intertidal species face daily and seasonal environmental variability, including temperature, oxygen, salinity, and nutritional changes. Increasing anthropogenic pressure can bring pollutants and pathogens as additional stressors. Surprisingly, C. gigas demonstrates impressive adaptability to most of these challenges. We explore how ROS production, antioxidant protection, redox signaling, and metabolic adjustments can shed light on how redox biology supports oyster survival in harsh conditions. The review provides (i) a brief summary of shared redox sensing processes in metazoan; (ii) an overview of unique characteristics of the C. gigas intertidal habitat and the suitability of this species as a model organism; (iii) insights into the redox biology of C. gigas, including ROS sources, signaling pathways, ROS-scavenging systems, and thiol-containing proteins; and examples of (iv) hot topics that are underdeveloped in bivalve research linking redox biology with immunometabolism, physioxia, and development. Given its plasticity to environmental changes, C. gigas is a valuable model for studying the role of redox biology in the adaptation to harsh habitats, potentially providing novel insights for basic and applied studies in marine and comparative biochemistry and physiology.

A Straightforward Method for the Isolation and Cultivation of Galleria mellonella Hemocytes

Galleria mellonella is an alternative animal model of infection. The use of this species presents a wide range of advantages, as its maintenance and rearing are both easy and inexpensive. Moreover, its use is considered to be more ethically acceptable than other models, it is conveniently sized for manipulation, and its immune system has multiple similarities with mammalian immune systems. Hemocytes are immune cells that help encapsulate and eliminate pathogens and foreign particles. All of these reasons make this insect a promising animal model. However, cultivating G. mellonella hemocytes in vitro is not straightforward and it has many difficult challenges. Here, we present a methodologically optimized protocol to establish and maintain a G. mellonella hemocyte primary culture. These improvements open the door to easily and quickly study the toxicity of nanoparticles and the interactions of particles and materials in an in vitro environment.

Status in molluscan cell line development in last one decade (2010–2020): impediments and way forward

Despite the attempts that have started since the 1960s, not even a single cell line of marine molluscs is available. Considering the vast contribution of marine bivalve aquaculture to the world economy, the prevailing viral threats, and the dismaying lack of advancements in molluscan virology, the requirement of a marine molluscan cell line is indispensable. This synthetic review discusses the obstacles in developing a marine molluscan cell line concerning the choice of species, the selection of tissue and decontamination, and cell culture media, with emphasis given on the current decade 2010-2020. Detailed accounts on the experiments on the virus cultivation in vitro and molluscan cell immortalization, with a brief note on the history and applications of the molluscan cell culture, are elucidated to give a holistic picture of the current status and future trends in molluscan cell line development.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10616-022-00539-x.

Biomaterials and Bioactive Natural Products from Marine Invertebrates: From Basic Research to Innovative Applications

Aquatic invertebrates are a major source of biomaterials and bioactive natural products that can find applications as pharmaceutics, nutraceutics, cosmetics, antibiotics, antifouling products and biomaterials. Symbiotic microorganisms are often the real producers of many secondary metabolites initially isolated from marine invertebrates; however, a certain number of them are actually synthesized by the macro-organisms. In this review, we analysed the literature of the years 2010–2019 on natural products (bioactive molecules and biomaterials) from the main phyla of marine invertebrates explored so far, including sponges, cnidarians, molluscs, echinoderms and ascidians, and present relevant examples of natural products of interest to public and private stakeholders. We also describe omics tools that have been more relevant in identifying and understanding mechanisms and processes underlying the biosynthesis of secondary metabolites in marine invertebrates. Since there is increasing attention on finding new solutions for a sustainable large-scale supply of bioactive compounds, we propose that a possible improvement in the biodiscovery pipeline might also come from the study and utilization of aquatic invertebrate stem cells.

Cell culture and genetic transfection methods for the Japanese scallop, Patinopecten yessoensis

Cell cultures can simplify assays of biological phenomena; therefore, cell culture systems have been established for many species, even invertebrates. However, there are few primary culture systems from marine invertebrates that can be maintained long term. The Japanese scallop, Patinopecten yessoensis, is a marine bivalve. Cell culture systems for the scallop have only been established for a few organ-derived cell types and for embryonic cells. We developed a primary culture system for cells from male and female scallop gonads, hepatopancreas, and adductor muscle by utilizing culture conditions closer to those in nature, with regard to temperature, osmolarity, and nutrition. Primary cultured female gonadal cells were maintained for more than 1 month and had potential for proliferation. Furthermore, a genetic transfection system was attempted using a scallop-derived promoter and a lipofection reagent. GFP-positive cells were detected in the attempt. These technical developments would promote our understanding of biochemical mechanisms in scallops as well as providing clues for establishment of immortalized molluscan cell lines.

Potential of genomic technologies to improve disease resistance in molluscan aquaculture

Molluscan aquaculture is a major contributor to global seafood production, but is hampered by infectious disease outbreaks that can cause serious economic losses. Selective breeding has been widely used to improve disease resistance in major agricultural and aquaculture species, and has clear potential in molluscs, albeit its commercial application remains at a formative stage. Advances in genomic technologies, especially the development of cost-efficient genomic selection, have the potential to accelerate genetic improvement. However, tailored approaches are required owing to the distinctive reproductive and life cycle characteristics of molluscan species. Transgenesis and genome editing, in particular CRISPR/Cas systems, have been successfully trialled in molluscs and may further understanding and improvement of genetic resistance to disease through targeted changes to the host genome. Whole-organism genome editing is achievable on a much greater scale compared to other farmed species, making genome-wide CRISPR screening approaches plausible. This review discusses the current state and future potential of selective breeding, genomic tools and genome editing approaches to understand and improve host resistance to infectious disease in molluscs. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

Establishment of primary cell culture of Ruditapes decussatus haemocytes for metal toxicity assessment

In ecotoxicology, in vitro testing on cell cultures represents an ideal alternative to in vivo strategies for emerging contaminants. These tests have limited use particularly with marine invertebrates like the clams Ruditapes decussatus. In the present study, a primary culture of R. decussatus haemocytes was realized for the first time in order to determine the effect of metals (copper, zinc, and cobalt) on haemocyte parameters like viability and phagocytosis. Results showed that (i) among the studied medium, the modified Leibovitz (L-15) is the best for R. decussatus haemocytes primary culture. (ii) The primary culture system used here represents a suitable in vitro model for assessing cytotoxic responses, (iii) a decrease of cell viability and phagocytosis after 24 h exposure to 100 μg mL^-1 CoSO₄ and an increase of phagocytosis after 24 h exposure to 50 μg mL^-1CuSO₄.