Regeneration of the radial nerve cord in a holothurian: a promising new model system for studying post-traumatic recovery in the adult nervous system

Evolution of glial cells: a non-bilaterian perspective

Nervous systems of bilaterian animals generally consist of two cell types: neurons and glial cells. Despite accumulating data about the many important functions glial cells serve in bilaterian nervous systems, the evolutionary origin of this abundant cell type remains unclear. Current hypotheses regarding glial evolution are mostly based on data from model bilaterians. Non-bilaterian animals have been largely overlooked in glial studies and have been subjected only to morphological analysis. Here, we provide a comprehensive overview of conservation of the bilateral gliogenic genetic repertoire of non-bilaterian phyla (Cnidaria, Placozoa, Ctenophora, and Porifera). We overview molecular and functional features of bilaterian glial cell types and discuss their possible evolutionary history. We then examine which glial features are present in non-bilaterians. Of these, cnidarians show the highest degree of gliogenic program conservation and may therefore be crucial to answer questions about glial evolution.

Ethical Considerations for Echinoderms: New Initiatives in Welfare

This paper explores the ethical considerations surrounding research on echinoderms, a group of invertebrates that has recently garnered attention in the scientific community. The importance of responsible animal handling and the need for an ethical framework that encompasses echinoderms are emphasized. The 3Rs principle, advocating for the replacement of conscious living vertebrates with non-sentient material in research, is discussed as a guiding tool in current animal research practices. As invertebrates are generally classified as non-sentient animals, the replacement dimension tends to favor them as prevalent models in experimental research. While it currently lacks the means to assess the mental states of invertebrates, there is undeniable evidence of social behavior in many species, suggesting that a lack of interactions with these organisms could potentially adversely affect their wellbeing. In the last few years, considerable progress has been made in developing an ethical framework that takes invertebrates into account, particularly cephalopods, crustaceans, and echinoderms. In this context, we discuss the development of a broader conceptual framework of 5Rs that includes responsibility and respect, which may guide practices ensuring welfare in echinoderms, even in the absence of any particular normative.

Echinoderm radial glia in adult cell renewal, indeterminate growth, and regeneration

Echinoderms are a phylum of marine deterostomes with a range of interesting biological features. One remarkable ability is their impressive capacity to regenerate most of their adult tissues, including the central nervous system (CNS). The research community has accumulated data that demonstrates that, in spite of the pentaradial adult body plan, echinoderms share deep similarities with their bilateral sister taxa such as hemichordates and chordates. Some of the new data reveal the complexity of the nervous system in echinoderms. In terms of the cellular architecture, one of the traits that is shared between the CNS of echinoderms and chordates is the presence of radial glia. In chordates, these cells act as the main progenitor population in CNS development. In mammals, radial glia are spent in embryogenesis and are no longer present in adults, being replaced with other neural cell types. In non-mammalian chordates, they are still detected in the mature CNS along with other types of glia. In echinoderms, radial glia also persist into the adulthood, but unlike in chordates, it is the only known glial cell type that is present in the fully developed CNS. The echinoderm radial glia is a multifunctional cell type. Radial glia forms the supporting scaffold of the neuroepithelium, exhibits secretory activity, clears up dying or damaged cells by phagocytosis, and, most importantly, acts as a major progenitor cell population. The latter function is critical for the outstanding developmental plasticity of the adult echinoderm CNS, including physiological cell turnover, indeterminate growth, and a remarkable capacity to regenerate major parts following autotomy or traumatic injury. In this review we summarize the current knowledge on the organization and function of the echinoderm radial glia, with a focus on the role of this cell type in adult neurogenesis.

Radial glia and radial glia-like cells: Their role in neurogenesis and regeneration

Radial glia is a cell type traditionally associated with the developing nervous system, particularly with the formation of cortical layers in the mammalian brain. Nonetheless, some of these cells, or closely related types, called radial glia-like cells are found in adult central nervous system structures, functioning as neurogenic progenitors in normal homeostatic maintenance and in response to injury. The heterogeneity of radial glia-like cells is nowadays being probed with molecular tools, primarily by the expression of specific genes that define cell types. Similar markers have identified radial glia-like cells in the nervous system of non-vertebrate organisms. In this review, we focus on adult radial glia-like cells in neurogenic processes during homeostasis and in response to injury. We highlight our results using a non-vertebrate model system, the echinoderm Holothuria glaberrima where we have described a radial glia-like cell that plays a prominent role in the regeneration of the holothurian central nervous system.

Neurogenesis during Brittle Star Arm Regeneration Is Characterised by a Conserved Set of Key Developmental Genes

Simple Summary

Injuries to the central nervous system most often lead to irreversible damage in humans. Brittle stars are marine animals related to sea stars and sea urchins, and are one of our closest evolutionary relatives among invertebrates. Extraordinarily, they can perfectly regenerate their nerves even after completely severing the nerve cord after arm amputation. Understanding what genes and cellular mechanisms are used for this natural repair process in the brittle star might lead to new insights to guide strategies for therapeutics to improve outcomes for central nervous system injuries in humans.

Abstract

Neural regeneration is very limited in humans but extremely efficient in echinoderms. The brittle star Amphiura filiformis can regenerate both components of its central nervous system as well as the peripheral system, and understanding the molecular mechanisms underlying this ability is key for evolutionary comparisons not only within the echinoderm group, but also wider within deuterostomes. Here we characterise the neural regeneration of this brittle star using a combination of immunohistochemistry, in situ hybridization and Nanostring nCounter to determine the spatial and temporal expression of evolutionary conserved neural genes. We find that key genes crucial for the embryonic development of the nervous system in sea urchins and other animals are also expressed in the regenerating nervous system of the adult brittle star in a hierarchic and spatio-temporally restricted manner.

Neuroecology beyond the brain: learning in Echinodermata

We propose an expansion of neuroecological comparisons to include the capabilities of brainless and non-neural organisms. We begin this enterprise by conducting a systematic search for studies on learning in echinoderms. Echinodermata are marine invertebrates comprising starfish, brittle stars, sea cucumbers, sea urchins, and sea lilies. Animals in this phylum lack any centralized brain and instead possess diffuse neural networks known as nerve nets. The learning abilities of these animals are of particular interest as, within the bilaterian clade, they are close evolutionary neighbors to chordates, a phylum whose members exhibit complex feats in learning and contain highly specialized brains. The learning capacities and limitations of echinoderms can inform the evolution of nervous systems and learning in Bilateria. We find evidence of both non-associative and associative learning (in the form of classical conditioning) in echinoderms, which was primarily focused on starfish. Additional evidence of learning is documented in brittle stars, sand dollars, and sea urchins. We then discuss the evolutionary significance of learning capabilities without a brain, the presence of embodied cognition across multiple groups, and compare the learning present in echinoderms with the impressive cognitive abilities documented in the oldest linage group within vertebrates (the major group within the phylum of chordates), fish.

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.

Molecular Aspects of Regeneration Mechanisms in Holothurians

Holothurians, or sea cucumbers, belong to the phylum Echinodermata. They show good regenerative abilities. The present review provides an analysis of available data on the molecular aspects of regeneration mechanisms in holothurians. The genes and signaling pathways activated during the asexual reproduction and the formation of the anterior and posterior parts of the body, as well as the molecular mechanisms that provide regeneration of the nervous and digestive systems, are considered here. Damage causes a strong stress response, the signs of which are recorded even at late regeneration stages. In holothurian tissues, the concentrations of reactive oxygen species and antioxidant enzymes increase. Furthermore, the cellular and humoral components of the immune system are activated. Extracellular matrix remodeling and Wnt signaling play a major role in the regeneration in holothurians. All available morphological and molecular data show that the dedifferentiation of specialized cells in the remnant of the organ and the epithelial morphogenesis constitute the basis of regeneration in holothurians. However, depending on the type of damage, the mechanisms of regeneration may differ significantly in the spatial organization of regeneration process, the involvement of different cell types, and the depth of reprogramming of their genome (dedifferentiation or transdifferentiation).

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.

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.

Metalloproteinase inhibitor GM6001 delays regeneration in holothurians

The effect of the GM6001 metalloproteinase inhibitor on the regeneration of ambulacral structures in Eupentacta fraudatrix has been investigated. Inhibition of proteinase activity exerts a marked effect on regeneration, being dependent on the time when GM6001 is injected. When administration of the inhibitor begins on day 3 post-injury, regeneration is completely abolished, and the animals die. This means that early activation of proteinases is crucial for triggering the regenerative process in holothurians. When GM6001 in first injected on day 7 post-injury, the regeneration rate decreases. However, this effect has proven to be reversible: when inhibition ceases, the regeneration resumes. The effect of the inhibitor is manifested as a retarded degradation of the extracellular matrix, the lack of cell dedifferentiation, and, probably, a slower cell migration. The gelatinase activity is detected in all the regenerating organs of E. fraudatrix. In the holothurian Cucumaria japonica, which is not capable of healing skin wounds and ambulacrum reparation, no gelatinase activity was observed at the site of damage. A suggestion is made that proteinases play an important role in regeneration in holothurians. The most probable morphogenesis regulators are matrix metalloproteinases with gelatinase activity.

Radial Glia in Echinoderms

Radial glial cells are crucial in vertebrate neural development and regeneration. It has been recently proposed that this neurogenic cell type might be older than the chordate lineage itself and might have been present in the last common deuterostome ancestor. Here, we summarize the results of recent studies on radial glia in echinoderms, a highly regenerative phylum of marine invertebrates with shared ancestry to chordates. We discuss the involvement of these cells in both homeostatic neurogenesis and post-traumatic neural regeneration, compare the features of radial glia in echinoderms and chordates to each other, and review the molecular mechanisms that control differentiation and plasticity of the echinoderm radial glia. Overall, studies on echinoderm radial glia provide a unique opportunity to understand the fundamental biology of this cell type from evolutionary and comparative perspectives.

Sewing up the wounds : The epithelial morphogenesis as a central mechanism of calcaronean sponge regeneration

Sponges (Porifera) demonstrate prominent regeneration abilities and possess a wide variety of mechanisms, used during this process. In the current study, we combined in vivo observations with histological, immunohistochemical, and ultrastructural technics to elucidate the fine cellular mechanisms of the regeneration in the calcareous sponge Leucosolenia cf. variabilis. The regeneration of Leucosolenia cf. variabilis ends within 4-6 days. The crucial step of the process is the formation of the transient regenerative membrane, formed by the epithelial morphogenesis-spreading of the intact exopinacoderm and choanoderm. The spreading of the choanoderm is accompanied by the transdifferentiation of the choanocytes. The regenerative membrane develops without any contribution of the mesohyl cells. Subsequently, the membrane gradually transforms into the body wall. The cell proliferation is neither affected nor contributes to the regeneration at any stage. Thus, Leucosolenia cf. variabilis regeneration relies on the remodeling of the intact tissues through the epithelial morphogenesis, accompanied by the transdifferentiation of some differentiated cell types, which makes it similar to the regeneration in homoscleromorphs and eumetazoans.

Getting a Head with Ptychodera flava Larval Regeneration

Severe injury to the central nervous system of chordates often results in permanent and irreversible mental and physical challenges. While some chordates are able to repair and/or regenerate portions of their nervous system, no chordate has been shown to be able to regenerate all regions of its central nervous system after catastrophic injury or amputation. Some hemichordates, on the other hand, are able to efficiently regenerate all neural structures, including their dorsal, hollow neural tube after complete ablation. Solitary hemichordates are marine acorn worms and a sister group to the echinoderms. The hemichordate Ptychodera flava progresses from a pelagic, feeding tornaria larva to a tripartite benthic worm with an anterior proboscis, a middle collar region, and a long posterior trunk. The adult worm regenerates all body parts when bisected in the trunk, but it was unknown whether the regeneration process was present in tornaria larvae. Now, we show that P. flava larvae are capable of robust regeneration after bisection through the sagittal, coronal, and axial planes. We also use antibody staining to show that the apical sensory organ regenerates a rich, serotonin-positive complex of cells within two weeks after amputation. Cells labeled with 5-ethynyl-2'-deoxyuridine confirm that regeneration is occurring through epimorphic processes as new cells are added at the cut site and throughout the regenerating tissue. This study verifies that P. flava larvae can be used for future functional studies aimed at identifying the genetic and morphological mechanisms controlling central nervous system regeneration in a stem deuterostome.

Molecular characterization and gene expression patterns of retinoid receptors, in normal and regenerating tissues of the sea cucumber, Holothuria glaberrima

Retinoic acid receptors (RAR) and retinoid X receptors (RXR) are ligand-mediated transcription factors that synchronize intricate signaling networks in metazoans. Dimer formation between these two nuclear receptors mediates the recruitment of co-regulatory complexes coordinating the progression of signaling cascades during developmental and regenerative events. In the present study we identified and characterized the receptors for retinoic acid in the sea cucumber Holothuria glaberrima; a model system capable of regenerative organogenesis during adulthood. Molecular characterizations revealed the presence of three isoforms of RAR and two of RXR as a consequence of alternative splicing events. Various analyses including: primary structure sequencing, phylogenetic analysis, protein domain prediction, and multiple sequence alignment further confirmed their identity. Semiquantitative reverse transcription PCR analysis of each receptor isoform herein identified showed that the retinoid receptors are expressed in all tissues sampled: the mesenteries, respiratory trees, muscles, gonads, and the digestive tract. During regenerative organogenesis two of the receptors (RAR-L and RXR-T) showed differential expression in the posterior segment while RAR-S is differentially expressed in the anterior segment of the intestine. This work presents the first description of the components relaying the signaling for retinoic acid within this model system.

Holothurians as a Model System to Study Regeneration

Echinoderms possess an incredible regenerative capacity. Within this phylum, holothurians, better known as sea cucumbers, can regenerate most of their internal and external organs. While regeneration has been studied in several species, the most recent and extensive studies have been done in the species Holothuria glaberrima, the focus of most of our discussion. This chapter presents the model system and integrates the work that has been done to determine the major steps that take place, during regeneration of the intestinal and nervous system, from wound healing to the reestablishment of original function. We describe the cellular and molecular events associated with the regeneration processes and also describe the techniques that have been used, discuss the results, and explain the gaps in our knowledge that remain. We expect that the information provided here paves the road for new and young investigators to continue the study of the amazing potential of regeneration in members of the Echinodermata and how these studies will shed some light into the mechanisms that are common to many regenerative processes.

Inhibition of cell proliferation does not slow down echinoderm neural regeneration

Background

Regeneration of the damaged central nervous system is one of the most interesting post-embryonic developmental phenomena. Two distinct cellular events have been implicated in supplying regenerative neurogenesis with cellular material – generation of new cells through cell proliferation and recruitment of already existing cells through cell migration. The relative contribution and importance of these two mechanisms is often unknown.

Methods

Here, we use the regenerating radial nerve cord (RNC) of the echinoderm Holothuria glaberrima as a model of extensive post-traumatic neurogenesis in the deuterostome central nervous system. To uncouple the effects of cell proliferation from those of cell migration, we treated regenerating animals with aphidicolin, a specific inhibitor of S-phase DNA replication. To monitor the effect of aphidicolin on DNA synthesis, we used BrdU immunocytochemistry. The specific radial glial marker ERG1 was used to label the regenerating RNC. Cell migration was tracked with vital staining with the lipophilic dye DiI.

Results

Aphidicolin treatment resulted in a significant 2.1-fold decrease in cell proliferation. In spite of this, the regenerating RNC in the treated animals did not differ in histological architecture, size and cell number from its counterpart in the control vehicle-treated animals. DiI labeling showed extensive cell migration in the RNC. Some cells migrated from as far as 2 mm away from the injury plane to contribute to the neural outgrowth.

Conclusions

We suggest that inhibition of cell division in the regenerating RNC of H. glaberrima is compensated for by recruitment of cells, which migrate into the RNC outgrowth from deeper regions of the neuroepithelium. Neural regeneration in echinoderms is thus a highly regulative developmental phenomenon, in which the size of the cell pool can be controlled either by cell proliferation or cell migration, and the latter can neutralize perturbations in the former.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-017-0196-y) contains supplementary material, which is available to authorized users.

The Diversity of Spine Synapses in Animals

Here we examine the structure of the various types of spine synapses throughout the animal kingdom. Based on available evidence, we suggest that there are two major categories of spine synapses: invaginating and non-invaginating, with distributions that vary among different groups of animals. In the simplest living animals with definitive nerve cells and synapses, the cnidarians and ctenophores, most chemical synapses do not form spine synapses. But some cnidarians have invaginating spine synapses, especially in photoreceptor terminals of motile cnidarians with highly complex visual organs, and also in some mainly sessile cnidarians with rapid prey capture reflexes. This association of invaginating spine synapses with complex sensory inputs is retained in the evolution of higher animals in photoreceptor terminals and some mechanoreceptor synapses. In contrast to invaginating spine synapse, non-invaginating spine synapses have been described only in animals with bilateral symmetry, heads and brains, associated with greater complexity in neural connections. This is apparent already in the simplest bilaterians, the flatworms, which can have well-developed non-invaginating spine synapses in some cases. Non-invaginating spine synapses diversify in higher animal groups. We also discuss the functional advantages of having synapses on spines and more specifically, on invaginating spines. And finally we discuss pathologies associated with spine synapses, concentrating on those systems and diseases where invaginating spine synapses are involved.

Skeletal regeneration in the brittle star Amphiura filiformis

Background

Brittle stars regenerate their whole arms post-amputation. Amphiura filiformis can now be used for molecular characterization of arm regeneration due to the availability of transcriptomic data. Previous work showed that specific developmental transcription factors known to take part in echinoderm skeletogenesis are expressed during adult arm regeneration in A. filiformis; however, the process of skeleton formation remained poorly understood. Here, we present the results of an in-depth microscopic analysis of skeletal morphogenesis during regeneration, using calcein staining, EdU labeling and in situ hybridization.

Results

To better compare different samples, we propose a staging system for the early A. filiformis arm regeneration stages based on morphological landmarks identifiable in living animals and supported by histological analysis. We show that the calcified spicules forming the endoskeleton first appear very early during regeneration in the dermal layer of regenerates. These spicules then mature into complex skeletal elements of the differentiated arm during late regeneration. The mesenchymal cells in the dermal area express the skeletal marker genes Afi-c-lectin, Afi-p58b and Afi-p19; however, EdU labeling shows that these dermal cells do not proliferate.

Conclusions

A. filiformis arms regenerate through a consistent set of developmental stages using a distalization-intercalation mode, despite variability in regeneration rate. Skeletal elements form in a mesenchymal cell layer that does not proliferate and thus must be supplied from a different source. Our work provides the basis for future cellular and molecular studies of skeleton regeneration in brittle stars.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-016-0149-x) contains supplementary material, which is available to authorized users.

Heterogeneous generation of new cells in the adult echinoderm nervous system

Adult neurogenesis, generation of new functional cells in the mature central nervous system (CNS), has been documented in a number of diverse organisms, ranging from humans to invertebrates. However, the origin and evolution of this phenomenon is still poorly understood for many of the key phylogenetic groups. Echinoderms are one such phylum, positioned as a sister group to chordates within the monophyletic clade Deuterostomia. They are well known for the ability of their adult organs, including the CNS, to completely regenerate after injury. Nothing is known, however, about production of new cells in the nervous tissue under normal physiological conditions in these animals. In this study, we show that new cells are continuously generated in the mature radial nerve cord (RNC) of the sea cucumber Holothuria glaberrima. Importantly, this neurogenic activity is not evenly distributed, but is significantly more extensive in the lateral regions of the RNC than along the midline. Some of the new cells generated in the apical region of the ectoneural neuroepithelium leave their place of origin and migrate basally to populate the neural parenchyma. Gene expression analysis showed that generation of new cells in the adult sea cucumber CNS is associated with transcriptional activity of genes known to be involved in regulation of various aspects of neurogenesis in other animals. Further analysis of one of those genes, the transcription factor Myc, showed that it is expressed, in some, but not all radial glial cells, suggesting heterogeneity of this CNS progenitor cell population in echinoderms.

Myc regulates programmed cell death and radial glia dedifferentiation after neural injury in an echinoderm

Background

Adult echinoderms can completely regenerate major parts of their central nervous system even after severe injuries. Even though this capacity has long been known, the molecular mechanisms that drive fast and complete regeneration in these animals have remained uninvestigated. The major obstacle for understanding these molecular pathways has been the lack of functional genomic studies on regenerating adult echinoderms.

Results

Here, we employ RNA interference-mediated gene knockdown to characterize the role of Myc during the early (first 48 hours) post-injury response in the radial nerve cord of the sea cucumber Holothuria glaberrima. Our previous experiments identified Myc as the only pluripotency-associated factor, whose expression significantly increased in the wounded CNS. The specific function(s) of this gene, however, remained unknown. Here we demonstrate that knockdown of Myc inhibits dedifferentiation of radial glia and programmed cell death, the two most prominent cellular events that take place in the regenerating sea cucumber nervous system shortly after injury.

Conclusions

In this study, we show that Myc overexpression is required for proper dedifferentiation of radial glial cells and for triggering the programmed cell death in the vicinity of the injury. Myc is thus the first transcription factor, whose functional role has been experimentally established in echinoderm regeneration.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0071-z) contains supplementary material, which is available to authorized users.

Expression of pluripotency factors in echinoderm regeneration

Cell dedifferentiation is an integral component of post-traumatic regeneration in echinoderms. As dedifferentiated cells become multipotent, we asked if this spontaneous broadening of developmental potential is associated with the action of the same pluripotency factors (known as Yamanaka factors) that were used to induce pluripotency in specialized mammalian cells. In this study, we investigate the expression of orthologs of the four Yamanaka factors in regeneration of two different organs, the radial nerve cord and the digestive tube, in the sea cucumber Holothuria glaberrima. All four pluripotency factors are expressed in uninjured animals, although their expression domains do not always overlap. In regeneration, the expression levels of the four genes were not regulated in a coordinated way, but instead showed different dynamics for individual genes and also were different between the radial nerve and the gut. SoxB1, the ortholog of the mammalian Sox2, was drastically downregulated in the regenerating intestine, suggesting that this factor is not required for dedifferentiation/regeneration in this organ. On the other hand, during the early post-injury stage, Myc, the sea cucumber ortholog of c-Myc, was significantly upregulated in both the intestine and the radial nerve cord and is therefore hypothesized to play a central role in dedifferentiation/regeneration of various tissue types.

Retrotransposons in animal regeneration: Overlooked components of the regenerative machinery?

Research on the involvement of retroelements in developmental processes has been gaining momentum recently; however, most of the studies published so far have been focused on embryonic development. This commentary presents two recent papers, which document significant changes in transcriptional activity of retroelements in two different model systems, salamander limb regeneration and regeneration of radial organs in the sea cucumber Holothuria glaberrima. We hypothesize that transcriptional activity of the retrotransposons can be specifically controlled by the host and may play some hitherto unrecognized role in regeneration.

Temporal and spatial analysis of enteric nervous system regeneration in the sea cucumber Holothuria glaberrima

There is limited information on the regeneration of the enteric nervous system (ENS) following major reconstruction of the digestive tract. We have studied ENS regeneration in the sea cucumber Holothuria glaberrima which undergoes an organogenic process forming a new digestive tract at the tip of the mesentery. Our results show that (1) a degeneration of nerve fibers occurs early in the regeneration process, prior to eventual regeneration; (2) nerve fibers that innervate the regenerating intestine are of extrinsic and intrinsic origin; (3) innervation by extrinsic fibers occurs in a gradient that begins in the proximal area of the regenerate; (4) late events include the appearance of nerve fibers that project from the serosa into the connective tissue and of nerve bundles in the mesothelial layer; (5) neurons and neuroendocrine cells appear early following the formation of the epithelial layers. Our results provide not only a comparative biological approach to study ENS regeneration but also an alternative point of view for the study of enteric neuropathologies and for the innervation of organs made in vitro.

Transcriptomic changes during regeneration of the central nervous system in an echinoderm

BACKGROUND

Echinoderms are emerging as important models in regenerative biology. Significant amount of data are available on cellular mechanisms of post-traumatic repair in these animals, whereas studies of gene expression are rare. In this study, we employ high-throughput sequencing to analyze the transcriptome of the normal and regenerating radial nerve cord (a homolog of the chordate neural tube), in the sea cucumber Holothuria glaberrima.

RESULTS

Our de novo assembly yielded 70,173 contigs, of which 24,324 showed significant similarity to known protein-coding sequences. Expression profiling revealed large-scale changes in gene expression (4,023 and 3,257 up-regulated and down-regulated transcripts, respectively) associated with regeneration. Functional analysis of sets of differentially expressed genes suggested that among the most extensively over-represented pathways were those involved in the extracellular matrix (ECM) remodeling and ECM-cell interactions, indicating a key role of the ECM in regeneration. We also searched the sea cucumber transcriptome for homologs of factors known to be involved in acquisition and/or control of pluripotency. We identified eleven genes that were expressed both in the normal and regenerating tissues. Of these, only Myc was present at significantly higher levels in regeneration, whereas the expression of Bmi-1 was significantly reduced. We also sought to get insight into which transcription factors may operate at the top of the regulatory hierarchy to control gene expression in regeneration. Our analysis yielded eleven putative transcription factors, which constitute good candidates for further functional studies. The identified candidate transcription factors included not only known regeneration-related genes, but also factors not previously implicated as regulators of post-traumatic tissue regrowth. Functional annotation also suggested that one of the possible adaptations contributing to fast and efficient neural regeneration in echinoderms may be related to suppression of excitotoxicity.

CONCLUSIONS

Our transcriptomic analysis corroborates existing data on cellular mechanisms implicated in regeneration in sea cucumbers. More importantly, however, it also illuminates new aspects of echinoderm regeneration, which have been scarcely studied or overlooked altogether. The most significant outcome of the present work is that it lays out a roadmap for future studies of regulatory mechanisms by providing a list of key candidate genes for functional analysis.

Radial glial cells play a key role in echinoderm neural regeneration

Background

Unlike the mammalian central nervous system (CNS), the CNS of echinoderms is capable of fast and efficient regeneration following injury and constitutes one of the most promising model systems that can provide important insights into evolution of the cellular and molecular events involved in neural repair in deuterostomes. So far, the cellular mechanisms of neural regeneration in echinoderm remained obscure. In this study we show that radial glial cells are the main source of new cells in the regenerating radial nerve cord in these animals.

Results

We demonstrate that radial glial cells of the sea cucumber Holothuria glaberrima react to injury by dedifferentiation. Both glia and neurons undergo programmed cell death in the lesioned CNS, but it is the dedifferentiated glial subpopulation in the vicinity of the injury that accounts for the vast majority of cell divisions. Glial outgrowth leads to formation of a tubular scaffold at the growing tip, which is later populated by neural elements. Most importantly, radial glial cells themselves give rise to new neurons. At least some of the newly produced neurons survive for more than 4 months and express neuronal markers typical of the mature echinoderm CNS.

Conclusions

A hypothesis is formulated that CNS regeneration via activation of radial glial cells may represent a common capacity of the Deuterostomia, which is not invoked spontaneously in higher vertebrates, whose adult CNS does not retain radial glial cells. Potential implications for biomedical research aimed at finding the cure for human CNS injuries are discussed.

Brachial plexus injury in adults: Diagnosis and surgical treatment strategies

Adult post traumatic Brachial plexus injury is unfortunately a rather common injury in young adults. In India the most common scenario is of a young man injured in a motorcycle accident. Exact incidence figures are not available but of the injuries presenting to us about 90% invole the above combination This article reviews peer-reviewed publications including clinical papers, review articles and Meta analysis of the subject. In addition, the authors' experience of several hundred cases over the last 15 years has been added and has influenced the ultimate text. Results have been discussed and analysed to get an idea of factors influencing final recovery. It appears that time from injury and number of roots involved are most crucial.

Organization of glial cells in the adult sea cucumber central nervous system

The nervous system of echinoderms has long been considered too unique to be directly comparable to the nervous system of other Deuterostomia. Using two novel monoclonal antibodies in combination with epifluorescence, confocal, and electron microscopy, we demonstrate here that the central nervous system of the sea cucumber Holothuria glaberrima possesses a major non-neuronal cell type, which shares striking similarities with the radial glia of chordates. The basic features in common include (a) an elongated shape, (b) long radial processes, (c) short lateral protrusions branching off the main processes and penetrating into the surrounding neuropile, (d) prominent orderly oriented bundles of intermediate filaments, and (e) ability to produce Reissner's substance. Radial glia account for the majority of glia cells in echinoderms and constitutes more than half of the total cell population in the radial nerve cord and about 45% in the circumoral nerve ring. The difference in glia cell number between those regions is significant, suggesting structural specialization within the seemingly simple echinoderm nervous system. Both cell death and proliferation are seen under normal physiological conditions. Although both glia and neurons undergo apoptosis, most of the mitotic cells are identified as radial glia, indicating a key role of this cell type in cell turnover in the nervous system. A hypothesis is proposed that the radial glia could be an ancestral feature of the deuterostome nervous system, and the origin of this cell type might have predated the diversification of the Chordata and Ambulacraria lineages.

Microscopic anatomy of the digestive system in normal and regenerating specimens of the brittlestar Amphipholis kochii

The morphology and regeneration of the digestive system of the ophiuroid Amphipholis kochii were investigated. The epithelia of the esophagus and stomach of A. kochii were composed of typical enterocytes and mucous cells. The digestive epithelium of the stomach contained two types of granular secretory cells. After autotomy of the disk, the animals retained the esophagus and a small part of the stomach. The dedifferentiation of enterocytes and mucous cells began on the first day after autotomy. On day 3 the cells formed an anlage of stomach around the mouth opening. Later, the stomach anlage grew as a result of cell proliferation. The opening on the aboral side of the body was closed by day 7. By this time differentiating cells were already observed in the stomach lining. The stomach mesothelium was formed by peritoneocytes and myoepithelial cells, which migrated from other coelomic epithelia of the body. Our study showed that the formation of the digestive system in A. kochii during regeneration depended on cells from the esophagus and the stomach remnant. Both enterocytes and mucous cells were able to dedifferentiate, migrate, and proliferate to give rise to the luminal epithelium. The basic mechanism of stomach formation was epithelial morphogenesis.

The central nervous system of sea cucumbers (Echinodermata: Holothuroidea) shows positive immunostaining for a chordate glial secretion

BACKGROUND

Echinoderms and chordates belong to the same monophyletic taxon, the Deuterostomia. In spite of significant differences in body plan organization, the two phyla may share more common traits than was thought previously. Of particular interest are the common features in the organization of the central nervous system. The present study employs two polyclonal antisera raised against bovine Reissner's substance (RS), a secretory product produced by glial cells of the subcomissural organ, to study RS-like immunoreactivity in the central nervous system of sea cucumbers.

RESULTS

In the ectoneural division of the nervous system, both antisera recognize the content of secretory vacuoles in the apical cytoplasm of the radial glia-like cells of the neuroepithelium and in the flattened glial cells of the non-neural epineural roof epithelium. The secreted immunopositive material seems to form a thin layer covering the cell apices. There is no accumulation of the immunoreactive material on the apical surface of the hyponeural neuroepithelium or the hyponeural roof epithelium. Besides labelling the supporting cells and flattened glial cells of the epineural roof epithelium, both anti-RS antisera reveal a previously unknown putative glial cell type within the neural parenchyma of the holothurian nervous system.

CONCLUSION

Our results show that: a) the glial cells of the holothurian tubular nervous system produce a material similar to Reissner's substance known to be synthesized by secretory glial cells in all chordates studied so far; b) the nervous system of sea cucumbers shows a previously unrealized complexity of glial organization. Our findings also provide significant clues for interpretation of the evolution of the nervous system in the Deuterostomia. It is suggested that echinoderms and chordates might have inherited the RS-producing radial glial cell type from the central nervous system of their common ancestor, i.e., the last common ancestor of all the Deuterostomia.

Regeneration of the radial nerve cord in the sea cucumber Holothuria glaberrima

Background

Regeneration of neurons and fibers in the mammalian spinal cord has not been plausible, even though extensive studies have been made to understand the restrictive factors involved. New experimental models and strategies are necessary to determine how new nerve cells are generated and how fibers regrow and connect with their targets in adult animals. Non-vertebrate deuterostomes might provide some answers to these questions. Echinoderms, with their amazing regenerative capacities could serve as model systems; however, very few studies have been done to study the regeneration of their nervous system.

Results

We have studied nerve cord regeneration in the echinoderm Holothuria glaberrima. These are sea cucumbers or holothurians members of the class Holothuroidea. One radial nerve cord, part of the echinoderm CNS, was completely transected using a scalpel blade. Animals were allowed to heal for up to four weeks (2, 6, 12, 20, and 28 days post-injury) before sacrificed. Tissues were sectioned in a cryostat and changes in the radial nerve cord were analyzed using classical dyes and immmuohistochemistry. In addition, the temporal and spatial distribution of cell proliferation and apoptosis was assayed using BrdU incorporation and the TUNEL assay, respectively.

We found that H. glaberrima can regenerate its radial nerve cord within a month following transection. The regenerated cord looks amazingly similar in overall morphology and cellular composition to the uninjured cord. The cellular events associated to radial cord regeneration include: (1) outgrowth of nerve fibers from the injured radial cord stumps, (2) intense cellular division in the cord stumps and in the regenerating radial nerve cords, (3) high levels of apoptosis in the RNC adjacent to the injury and within the regenerating cord and (4) an increase in the number of spherule-containing cells. These events are similar to those that occur in other body wall tissues during wound healing and during regeneration of the intestine.

Conclusion

Our data indicate that holothurians are capable of rapid and complete regeneration of the main component of their CNS. Regeneration involves both the outgrowth of nerve fibers and the formation of neurons. Moreover, the cellular events employed during regeneration are similar to those involved in other regenerative processes, namely wound healing and intestinal regeneration. Thus, holothurians should be viewed as an alternative model where many of the questions regarding nervous system regeneration in deuterostomes could be answered.