A Novel Approach to Primary Cell Culture for Octopus vulgaris Neurons

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.

Research Trends in Octopus Biological Studies

Simple Summary

Octopuses represent model studies for different fields of scientific inquiry. We provide a bibliometric analysis on biological research trends in octopuses studies by using bibliometrix, a new and powerful R-tool. The analysis was executed from January 1985 to December 2020 including scientific products reported in Web of Science (WoS) database. The main results showed an increasing effort in research involving octopuses with a greater number of journals reporting research on these animals, as well as countries, institutions, and researchers involved. Some research themes lost importance over time, while some new themes appeared recently. Current data provide significant insight into the evolving trends in octopuses studies.

Abstract

Octopuses represent interesting model studies for different fields of scientific inquiry. The present study provides a bibliometric analysis on research trends in octopuses biological studies. The analysis was executed from January 1985 to December 2020 including scientific products reported in the Web of Science database. The period of study was split into two blocks (“earlier period” (EP): 1985−2010; “recent period” (RP): 2011−2020) to analyze the evolution of the research topics over time. All publications of interest were identified by using the following query: ((AK = octopus) OR (AB = octopus) OR (TI = octopus)). Data information was converted into an R-data frame using bibliometrix. Octopuses studies appeared in 360 different sources in EP, while they increased to 408 in RP. Sixty countries contributed to the octopuses studies in the EP, while they were 78 in the RP. The number of affiliations also increased between EP and RP, with 835 research centers involved in the EP and 1399 in the RP. In the EP 5 clusters (i.e., “growth and nutrition”, “pollution impact”, “morphology”, “neurobiology”, “biochemistry”) were represented in a thematic map, according to their centrality and density ranking. In the RP the analysis identified 4 clusters (i.e., “growth and nutrition”, “ecology”, “pollution impact”, “genes, behavior, and brain evolution”). The UK with Ireland, and the USA with Canada shared the highest number of publications in the EP, while in the RP, Spain and Portugal were the leading countries. The current data provide significant insight into the evolving trends in octopuses studies.

Neurons and Glia Cells in Marine Invertebrates: An Update

The nervous system (NS) of invertebrates and vertebrates is composed of two main types of cells: neurons and glia. In both types of organisms, nerve cells have similarities in biochemistry and functionality. The neurons are in charge of the synapse, and the glial cells are in charge of important functions of neuronal and homeostatic modulation. Knowing the mechanisms by which NS cells work is important in the biomedical area for the diagnosis and treatment of neurological disorders. For this reason, cellular and animal models to study the properties and characteristics of the NS are always sought. Marine invertebrates are strategic study models for the biological sciences. The sea slug Aplysia californica and the squid Loligo pealei are two examples of marine key organisms in the neurosciences field. The principal characteristic of marine invertebrates is that they have a simpler NS that consists of few and larger cells, which are well organized and have accessible structures. As well, the close phylogenetic relationship between Chordata and Echinodermata constitutes an additional advantage to use these organisms as a model for the functionality of neuronal cells and their cellular plasticity. Currently, there is great interest in analyzing the signaling processes between neurons and glial cells, both in vertebrates and in invertebrates. However, only few types of glial cells of invertebrates, mostly insects, have been studied, and it is important to consider marine organisms' research. For this reason, the objective of the review is to present an update of the most relevant information that exists around the physiology of marine invertebrate neuronal and glial cells.

Sea urchin Paracentrotus lividus immune cells in culture: formulation of the appropriate harvesting and culture media and maintenance conditions

The sea urchin is an emergent model system for studying basic and translational immunology. Here we report a new method for the harvesting and maintenance of primary immune cells isolated from adult Paracentrotus lividus, a common Mediterranean sea urchin species. This optimised method uses coelomocyte culture medium, containing a high-affinity Ca²⁺ chelator, as the ideal harvesting and anti-clotting vehicle and short-term culture medium (≤48 h), and artificial seawater as the master medium that maintains cell survival and in vitro-ex vivo physiological homeostasis over 2 weeks. Gradually reducing the amount of anticoagulant solution in the medium and regularly replacing the medium led to improved culture viability. Access to a robust and straightforward in vitro-ex vivo system will expedite our understanding of deuterostome immunity as well as underscore the potential of sea urchin with respect to biomedicine and regulatory testing.

This article has an associated First Person interview with the first author of the paper.

Summary: Appropriate culture methods for sea urchin immune cells provide an invaluable and amenable model for answering immunological questions while limiting the use of mammalian organisms.

Sense and Insensibility - An Appraisal of the Effects of Clinical Anesthetics on Gastropod and Cephalopod Molluscs as a Step to Improved Welfare of Cephalopods

Recent progress in animal welfare legislation stresses the need to treat cephalopod molluscs, such as Octopus vulgaris, humanely, to have regard for their wellbeing and to reduce their pain and suffering resulting from experimental procedures. Thus, appropriate measures for their sedation and analgesia are being introduced. Clinical anesthetics are renowned for their ability to produce unconsciousness in vertebrate species, but their exact mechanisms of action still elude investigators. In vertebrates it can prove difficult to specify the differences of response of particular neuron types given the multiplicity of neurons in the CNS. However, gastropod molluscs such as Aplysia, Lymnaea, or Helix, with their large uniquely identifiable nerve cells, make studies on the cellular, subcellular, network and behavioral actions of anesthetics much more feasible, particularly as identified cells may also be studied in culture, isolated from the rest of the nervous system. To date, the sorts of study outlined above have never been performed on cephalopods in the same way as on gastropods. However, criteria previously applied to gastropods and vertebrates have proved successful in developing a method for humanely anesthetizing Octopus with clinical doses of isoflurane, i.e., changes in respiratory rate, color pattern and withdrawal responses. However, in the long term, further refinements will be needed, including recordings from the CNS of intact animals in the presence of a variety of different anesthetic agents and their adjuvants. Clues as to their likely responsiveness to other appropriate anesthetic agents and muscle relaxants can be gained from background studies on gastropods such as Lymnaea, given their evolutionary history.

The Cephalopod Large Brain Enigma: Are Conserved Mechanisms of Stem Cell Expansion the Key?

Within the clade of mollusks, cephalopods have developed an unusually large and complex nervous system. The increased complexity of the cephalopod centralized "brain" parallels an amazing amount of complex behaviors that culminate in one order, the octopods. The mechanisms that enable evolution of expanded brains in invertebrates remain enigmatic. While expression mapping of known molecular pathways demonstrated the conservation of major neurogenesis pathways and revealed neurogenic territories, it did not explain why cephalopods could massively increase their brain size compared to other mollusks. Such an increase is reminiscent of the expansion of the cerebral cortex in mammalians, which have enlarged their number and variety of neurogenic stem cells. We hypothesize that similar mechanisms might be at play in cephalopods and that focusing on the stem cell biology of cephalopod neurogenesis and genetic innovations might be smarter strategies to uncover the mechanism that has driven cephalopod brain expansion.

NeuroSystematics and Periodic System of Neurons: Model vs Reference Species at Single-Cell Resolution

There is more than one way to develop neuronal complexity, and animals frequently use different molecular toolkits to achieve similar functional outcomes (=convergent evolution). Neurons are different not only because they have different functions, but also because neurons and circuits have different genealogies, and perhaps independent origins at the broadest scale from ctenophores and cnidarians to cephalopods and primates. By combining modern phylogenomics, single-neuron sequencing (scRNA-seq), machine learning, single-cell proteomics, and metabolomic across Metazoa, it is possible to reconstruct the evolutionary histories of neurons tracing them to ancestral secretory cells. Comparative data suggest that neurons, and perhaps synapses, evolved at least 2-3 times (in ctenophore, cnidarian and bilateral lineages) during ∼600 million years of animal evolution. There were also several independent events of the nervous system centralization either from a common bilateral/cnidarian ancestor without the bona fide neurons or from the urbilaterian with diffuse, nerve-net type nervous system. From the evolutionary standpoint, (i) a neuron should be viewed as a functional rather than a genetic character, and (ii) any given neural system might be chimeric and composed of different cell lineages with distinct origins and evolutionary histories. The identification of distant neural homologies or examples of convergent evolution among 34 phyla will not only allow the reconstruction of neural systems' evolution but together with single-cell "omic" approaches the proposed synthesis would lead to the "Periodic System of Neurons" with predictive power for neuronal phenotypes and plasticity. Such a phylogenetic classification framework of Neuronal Systematics (NeuroSystematics) might be a conceptual analog of the Periodic System of Chemical Elements. scRNA-seq profiling of all neurons in an entire brain or Brain-seq is now fully achievable in many nontraditional reference species across the entire animal kingdom. Arguably, marine animals are the most suitable for the proposed tasks because the world oceans represent the greatest taxonomic and body-plan diversity.

Magnitude Assessment of Adult Neurogenesis in the Octopus vulgaris Brain Using a Flow Cytometry-Based Technique

Adult neurogenesis is widespread among metazoans, it occurs in animals with a network nervous system, as cnidarians, and in animals with a complex and centralized brain, such as mammals, non-mammalian vertebrates, ecdysozoans, and a lophotrochozoan, Octopus vulgaris. Nevertheless, there are important differences among taxa, especially in the number of the regions involved and in cell proliferation rate during the life-cycle. The comparative evaluation of adult neurogenesis among different brain regions is an arduous task to achieve with only stereological techniques. However, in Octopus vulgaris we recently confirmed the presence of active proliferation in the learning-memory centers, multisensory integration centers, and the motor centers of the adult brain. Here, using a flow cytometry technique, we provide a method to quantify the active proliferation in octopus nervous system using a BrdU in vitro administration without exposing the animals to stress or painful injections usually used. This method is in line with the current animal welfare regulations regarding cephalopods, and the flow cytometry-based technique enabled us to measure adult neurogenesis more quickly and reliably than histological techniques, with the additional advantage of processing multiple samples in parallel. Flow cytometry is thus an appropriate technique for measuring and comparing adult neurogenesis in animals that are in a different physiological and/or environmental contexts. A BrdU immunoreactivity distribution, to define the neurogenic areas, and the effective penetration in vitro of the BrdU is also provided.

The Current State of Cephalopod Science and Perspectives on the Most Critical Challenges Ahead From Three Early-Career Researchers

Here, three researchers who have recently embarked on careers in cephalopod biology discuss the current state of the field and offer their hopes for the future. Seven major topics are explored: genetics, aquaculture, climate change, welfare, behavior, cognition, and neurobiology. Recent developments in each of these fields are reviewed and the potential of emerging technologies to address specific gaps in knowledge about cephalopods are discussed. Throughout, the authors highlight specific challenges that merit particular focus in the near-term. This review and prospectus is also intended to suggest some concrete near-term goals to cephalopod researchers and inspire those working outside the field to consider the revelatory potential of these remarkable creatures.