Echinoderm regeneration: an in vitro approach using the crinoid Antedon mediterranea

Rainbow bodies: Revisiting the diversity of coelomocyte aggregates and their synthesis in echinoderms

The innate immunity of echinoderms has been a research focus since the early twentieth century, consistently providing ever deeper knowledge of its complexity and evolutionary aspects. At its core are coelomocytes, which are diverse cells collectively known to respond in a variety of ways, including via movement, phagocytosis, and aggregation. However, features of cellular immunity have never been compared in echinoderms from phylogenetic and distributional perspectives, to provide insight into ecological and evolutionary patterns. The present study catalyzed and characterized the formation of coelomocyte aggregates in members of all five extant classes of echinoderms. The morphological characteristics of these aggregates (including their colour, shape, texture, size) were assessed, as well as the major cells composing them. Coelomocyte diversity (both as free and aggregated forms) was determined to be maximum in class Holothuroidea, followed by Echinoidea, with the other classes showing similar levels of diversity. The colours of coelomocyte aggregates appeared to be more closely linked to phylogeny (classes, orders) rather than geographic range, or external colour of the species themselves. Asteroids and ophiuroids displayed primarily light-coloured aggregates, from transparent to green; while holothuroids, echinoids and crinoids demonstrated more vivid variants, from red to deep purple. The kinetics of aggregate formation and expulsion were monitored in selected species, showing immediate cellular response to foreign particulate matter in the form of encapsulation and various methods of expulsion, including through the dermal papillae of asteroids and the anus (cloaca) of holothuroids. The findings support that coelomocyte aggregate formation is a conserved immune response across all five extant classes of echinoderms with variations in their cell catalysts, complexity, shape, colour, and size.

Intestine Explants in Organ Culture: A Tool to Broaden the Regenerative Studies in Echinoderms

The cellular events underlying intestine regrowth in the sea cucumber Holothuria glaberrima have been described by our group. Currently, the molecular and signaling mechanisms involved in this process are being explored. One of the limitations to our investigations has been the absence of suitable cell culture methodologies, required to advance the regeneration studies. An in vitro system, where regenerating intestine explants can be studied in organ culture, was established previously by our group. However, a detailed description of the histological properties of the cultured gut explants was lacking. Here, we used immunocytochemical techniques to study the potential effects of the culture conditions on the histological characteristics of explants, comparing them to the features observed during gut regeneration in our model in vivo. Additionally, the explant outgrowths were morphologically described by phase-contrast microscopy and SEM. Remarkably, intestine explants retain most of their original histoarchitecture for up to 10 days, with few changes as culture time increases. The most evident effects of the culture conditions on explants over culture time were the reduction in the proliferative rate, the loss of the polarity in the localization of proliferating cells, and the appearance of a subpopulation of putative spherulocytes. Finally, cells that migrated from the gut explants could form net-like monolayers, firmly attached to the culture substrate. Overall, regenerating explants in organ culture represent a powerful tool to perform short-term studies of processes associated with gut regeneration in H. glaberrima under controlled conditions.

Dedifferentiation of radial glia-like cells is observed in in vitro explants of holothurian radial nerve cord

BACKGROUND

Among animal phyla, some of the least studied nervous systems are those of the phylum Echinodermata. Part of the problem lies in that most of their nervous components are embedded in the body wall that has calcareous skeletal components.

NEW METHOD

We have developed a novel technique for the successful isolation of the radial nerve cords (RNCs) and an in vitro system where the isolated RNCs can be cultured and are amenable to experimental manipulation. Here we use this system to isolate the RNC of the sea cucumber Holothuria glaberrima as a way to extend our studies on its regeneration capabilities.

RESULTS

The RNCs can be isolated from the surrounding tissues by collagenase treatment. The explants obtained following enzymatic dissociation can be kept in culture for up to 2 weeks. Histological and immunohistochemical studies show that the explants maintain a stable number of cells with little proliferation or apoptosis throughout the culture incubation period. The main change observed in RNCs in vitro is a progressive dedifferentiation of radial glia-like cells. This dedifferentiation corresponds to the first step in the regeneration response to injury that has been described in vivo.

COMPARISON WITH EXISTING METHODS

There are no existing methods to isolate and culture echinoderm radial nerve cord.

CONCLUSIONS

The described protocol provides a unique tool to obtain easily accessible RNC from holothurians to perform cellular, biochemical, and genomic experiments in the echinoderm nervous system without interference of adjacent tissues. The technique provides a unique opportunity to study the dedifferentiation response associated with the regeneration of the nervous system in echinoderms.

A pan-metazoan concept for adult stem cells: the wobbling Penrose landscape

Adult stem cells (ASCs) in vertebrates and model invertebrates (e.g. Drosophila melanogaster) are typically long‐lived, lineage‐restricted, clonogenic and quiescent cells with somatic descendants and tissue/organ‐restricted activities. Such ASCs are mostly rare, morphologically undifferentiated, and undergo asymmetric cell division. Characterized by ‘stemness’ gene expression, they can regulate tissue/organ homeostasis, repair and regeneration. By contrast, analysis of other animal phyla shows that ASCs emerge at different life stages, present both differentiated and undifferentiated phenotypes, and may possess amoeboid movement. Usually pluri/totipotent, they may express germ‐cell markers, but often lack germ‐line sequestering, and typically do not reside in discrete niches. ASCs may constitute up to 40% of animal cells, and participate in a range of biological phenomena, from whole‐body regeneration, dormancy, and agametic asexual reproduction, to indeterminate growth. They are considered legitimate units of selection. Conceptualizing this divergence, we present an alternative stemness metaphor to the Waddington landscape: the ‘wobbling Penrose’ landscape. Here, totipotent ASCs adopt ascending/descending courses of an ‘Escherian stairwell’, in a lifelong totipotency pathway. ASCs may also travel along lower stemness echelons to reach fully differentiated states. However, from any starting state, cells can change their stemness status, underscoring their dynamic cellular potencies. Thus, vertebrate ASCs may reflect just one metazoan ASC archetype.

The Use of Larval Sea Stars and Sea Urchins in the Discovery of Shared Mechanisms of Metazoan Whole-Body Regeneration

The ability to regenerate is scattered among the metazoan tree of life. Further still, regenerative capacity varies widely within these specific organisms. Numerous organisms, all with different regenerative capabilities, have been studied at length and key similarities and disparities in how regeneration occurs have been identified. In order to get a better grasp on understanding regeneration as a whole, we must search for new models that are capable of extensive regeneration, as well as those that have been under sampled in the literature. As invertebrate deuterostomes, echinoderms fit both of these requirements. Multiple members regenerate various tissue types at all life stages, including examples of whole-body regeneration. Interrogations in two highly studied echinoderms, the sea urchin and the sea star, have provided knowledge of tissue and whole-body regeneration at various life stages. Work has begun to examine regeneration in echinoderm larvae, a potential new system for understanding regenerative mechanisms in a basal deuterostome. Here, we review the ways these two animals’ larvae have been utilized as a model of regeneration.

Animal regeneration in the era of transcriptomics

Animal regeneration, the ability to restore a lost body part, is a process that has fascinated scientists for centuries. In this review, we first present what regeneration is and how it relates to development, as well as the widespread and diverse nature of regeneration in animals. Despite this diversity, animal regeneration includes three common mechanistic steps: initiation, induction and activation of progenitors, and morphogenesis. In this review article, we summarize and discuss, from an evolutionary perspective, the recent data obtained for a variety of regeneration models which have allowed to identify key shared mechanisms that control these main steps of animal regeneration. This review also synthesizes the wealth of high-throughput mRNA sequencing data (bulk mRNA-seq) concerning regeneration which have been obtained in recent years, highlighting the major advances in the regeneration field that these studies have revealed. We stress out that, through a comparative approach, these data provide opportunities to further shed light on the evolution of regeneration in animals. Finally, we point out how the use of single-cell mRNA-seq technology and integration with epigenomic approaches may further help researchers to decipher mechanisms controlling regeneration and their evolution in animals.

Beyond Adult Stem Cells: Dedifferentiation as a Unifying Mechanism Underlying Regeneration in Invertebrate Deuterostomes

The diversity of regenerative phenomena seen in adult metazoans, as well as their underlying mechanistic bases, are still far from being comprehensively understood. Reviewing both ultrastructural and molecular data, the present work aims to showcase the increasing relevance of invertebrate deuterostomes, i.e., echinoderms, hemichordates, cephalochordates and tunicates, as invaluable models to study cellular aspects of adult regeneration. Our comparative approach suggests a fundamental contribution of local dedifferentiation -rather than mobilization of resident undifferentiated stem cells- as an important cellular mechanism contributing to regeneration in these groups. Thus, elucidating the cellular origins, recruitment and fate of cells, as well as the molecular signals underpinning tissue regrowth in regeneration-competent deuterostomes, will provide the foundation for future research in tackling the relatively limited regenerative abilities of vertebrates, with clear applications in regenerative medicine.

Nervous system characterization during the development of a basal echinoderm, the feather star Antedon mediterranea

Neural development of echinoderms has always been difficult to interpret, as larval neurons degenerate at metamorphosis and a tripartite nervous system differentiates in the adult. Despite their key phylogenetic position as basal echinoderms, crinoids have been scarcely studied in developmental research. However, since they are the only extant echinoderms retaining the ancestral body plan of the group, crinoids are extremely valuable models to clarify neural evolution in deuterostomes. Antedon mediterranea is a feather star, endemic to the Mediterranean Sea. Its development includes a swimming lecithotrophic larva, the doliolaria, with basiepithelial nerve plexus, and a sessile filter-feeding juvenile, the pentacrinoid, whose nervous system has never been described in detail. Thus, we characterized the nervous system of both these developmental stages by means of immunohistochemistry and, for the first time, in situ hybridization techniques. The results confirmed previous descriptions of doliolaria morphology and revealed that the larval apical organ contains two bilateral clusters of serotonergic cells while GABAergic neurons are localized under the adhesive pit. This suggested that different larval activities (e.g., attachment and metamorphosis) are under the control of different neural populations. In pentacrinoids, the analysis showed the presence of a cholinergic entoneural system while the ectoneural plexus appeared more composite, displaying different neural populations. The expression of three neural-related microRNAs was described for the first time, suggesting that these are evolutionarily conserved also in basal echinoderms. Overall, our results set the stage for future investigations that will reveal new information on echinoderm evo-devo neurobiology.

Regeneration in Stellate Echinoderms: Crinoidea, Asteroidea and Ophiuroidea

Reparative regeneration is defined as the replacement of lost adult body parts and is a phenomenon widespread yet highly variable among animals. This raises the question of which key cellular and molecular mechanisms have to be implemented in order to efficiently and correctly replace entire body parts in any animal. To address this question, different studies using an integrated cellular and functional genomic approach to study regeneration in stellate echinoderms (crinoids, asteroids and ophiuroids) had been carried out over the last few years. The phylum Echinodermata is recognized for the striking regeneration potential shown by the members of its different clades. Indeed, stellate echinoderms are considered among the most useful and tractable experimental models for carrying comprehensive studies focused on ecological, developmental and evolutionary aspects. Moreover, most of them are tractable in the laboratory and, thus, should allow us to understand the underlying mechanisms, cellular and molecular, which are involved. Here, a comprehensive analysis of the cellular/histological components of the regenerative process in crinoids, asteroids and ophiuroids is described and compared. However, though this knowledge provided us with some clear insights into the global distribution of cell types at different times, it did not explain us how the recruited cells are specified (and from which precursors) over time and where are they located in the animal. The precise answer to these queries needs the incorporation of molecular approaches, both descriptive and functional. Yet, the molecular studies in stellate echinoderms are still limited to characterization of some gene families and protein factors involved in arm regeneration but, at present, have not shed light on most of the basic mechanisms. In this context, further studies are needed specifically to understand the role of regulatory factors and their spatio-temporal deployment in the growing arms. A focus on developing functional tools over the next few years should be of fundamental importance.

Collagenous Extracellular Matrix Biomaterials for Tissue Engineering: Lessons from the Common Sea Urchin Tissue

Scaffolds for tissue engineering application may be made from a collagenous extracellular matrix (ECM) of connective tissues because the ECM can mimic the functions of the target tissue. The primary sources of collagenous ECM material are calf skin and bone. However, these sources are associated with the risk of having bovine spongiform encephalopathy or transmissible spongiform encephalopathy. Alternative sources for collagenous ECM materials may be derived from livestock, e.g., pigs, and from marine animals, e.g., sea urchins. Collagenous ECM of the sea urchin possesses structural features and mechanical properties that are similar to those of mammalian ones. However, even more intriguing is that some tissues such as the ligamentous catch apparatus can exhibit mutability, namely rapid reversible changes in the tissue mechanical properties. These tissues are known as mutable collagenous tissues (MCTs). The mutability of these tissues has been the subject of on-going investigations, covering the biochemistry, structural biology and mechanical properties of the collagenous components. Recent studies point to a nerve-control system for regulating the ECM macromolecules that are involved in the sliding action of collagen fibrils in the MCT. This review discusses the key attributes of the structure and function of the ECM of the sea urchin ligaments that are related to the fibril-fibril sliding action-the focus is on the respective components within the hierarchical architecture of the tissue. In this context, structure refers to size, shape and separation distance of the ECM components while function is associated with mechanical properties e.g., strength and stiffness. For simplicity, the components that address the different length scale from the largest to the smallest are as follows: collagen fibres, collagen fibrils, interfibrillar matrix and collagen molecules. Application of recent theories of stress transfer and fracture mechanisms in fibre reinforced composites to a wide variety of collagen reinforcing (non-mutable) connective tissue, has allowed us to draw general conclusions concerning the mechanical response of the MCT at specific mechanical states, namely the stiff and complaint states. The intent of this review is to provide the latest insights, as well as identify technical challenges and opportunities, that may be useful for developing methods for effective mechanical support when adapting decellularised connective tissues from the sea urchin for tissue engineering or for the design of a synthetic analogue.