Evolution of the chordate regeneration blastema: Differential gene expression and conserved role of notch signaling during siphon regeneration in the ascidian Ciona

Conserved Signaling Pathways in the Ciona robusta Gut

The urochordate Ciona robusta exhibits numerous functional and morphogenetic traits that are shared with vertebrate models. While prior investigations have identified several analogies between the gastrointestinal tract (i.e., gut) of Ciona and mice, the molecular mechanisms responsible for these similarities remain poorly understood. This study seeks to address this knowledge gap by investigating the transcriptional landscape of the adult stage gut. Through comparative genomics analyses, we identified several evolutionarily conserved components of signaling pathways of pivotal importance for gut development (such as WNT, Notch, and TGFβ-BMP) and further evaluated their expression in three distinct sections of the gastrointestinal tract by RNA-seq. Despite the presence of lineage-specific gene gains, losses, and often unclear orthology relationships, the investigated pathways were characterized by well-conserved molecular machinery, with most components being expressed at significant levels throughout the entire intestinal tract of C. robusta. We also showed significant differences in the transcriptional landscape of the stomach and intestinal tract, which were much less pronounced between the proximal and distal portions of the intestine. This study confirms that C. robusta is a reliable model system for comparative studies, supporting the use of ascidians as a model to study gut physiology.

Differentially expressed chaperone genes reveal a stress response required for unidirectional regeneration in the basal chordate Ciona

BACKGROUND

Unidirectional regeneration in the basal chordate Ciona intestinalis involves the proliferation of adult stem cells residing in the branchial sac vasculature and the migration of progenitor cells to the site of distal injury. However, after the Ciona body is bisected, regeneration occurs in the proximal but not in the distal fragments, even if the latter include a part of the branchial sac with stem cells. A transcriptome was sequenced and assembled from the isolated branchial sacs of regenerating animals, and the information was used to provide insights into the absence of regeneration in distal body fragments.

RESULTS

We identified 1149 differentially expressed genes, which were separated into two major modules by weighted gene correlation network analysis, one consisting of mostly upregulated genes correlated with regeneration and the other consisting of only downregulated genes associated with metabolism and homeostatic processes. The hsp70, dnaJb4, and bag3 genes were among the highest upregulated genes and were predicted to interact in an HSP70 chaperone system. The upregulation of HSP70 chaperone genes was verified and their expression confirmed in BS vasculature cells previously identified as stem and progenitor cells. siRNA-mediated gene knockdown showed that hsp70 and dnaJb4, but not bag3, are required for progenitor cell targeting and distal regeneration. However, neither hsp70 nor dnaJb4 were strongly expressed in the branchial sac vasculature of distal fragments, implying the absence of a stress response. Heat shock treatment of distal body fragments activated hsp70 and dnaJb4 expression indicative of a stress response, induced cell proliferation in branchial sac vasculature cells, and promoted distal regeneration.

CONCLUSIONS

The chaperone system genes hsp70, dnaJb4, and bag3 are significantly upregulated in the branchial sac vasculature following distal injury, defining a stress response that is essential for regeneration. The stress response is absent from distal fragments, but can be induced by a heat shock, which activates cell division in the branchial sac vasculature and promotes distal regeneration. This study demonstrates the importance of a stress response for stem cell activation and regeneration in a basal chordate, which may have implications for understanding the limited regenerative activities in other animals, including vertebrates.

Studying Regeneration in Ascidians: An Historical Overview

Ascidians are sessile tunicates, that is, marine animals belonging to the phylum Chordata and considered the sister group of vertebrates. They are widespread in all the seas, constituting abundant communities in various ecosystems. Among chordates, only tunicates are able to reproduce asexually, forming colonies. The high regenerative potentialities enabling tunicates to regenerate damaged body parts, or the whole body, represent a peculiarity of this taxon. Here we review the methodological approaches used in more than a century of biological studies to induce regeneration in both solitary and colonial species. For solitary species, we refer to the regeneration of single organs or body parts (e.g., siphon, brain, gonad, tunic, viscera). For colonial species, we review a plethora of experiments regarding the surgical manipulation of colonies, the regeneration of isolated colonial entities, such as single buds in the tunic, or part of tunic and its circulatory system.

Apoptosis is a generator of Wnt-dependent regeneration and homeostatic cell renewal in the ascidian Ciona

In the ascidian Ciona intestinalis, basal body parts regenerate distal structures but distal body parts do not replace basal structures. Regeneration involves the activity of adult stem cells in the branchial sac, which proliferate and produce migratory progenitor cells for tissue and organ replacement. Branchial sac-derived stem cells also replenish recycling cells lining the pharyngeal fissures during homeostatic growth. Apoptosis at injury sites occurs early during regeneration and continuously in the pharyngeal fissures during homeostatic growth. Caspase 1 inhibitor, caspase 3 inhibitor, or pan-caspase inhibitor Z-VAD-FMK treatment blocked apoptosis, prevented regeneration, and suppressed branchial sac growth and function. A pharmacological screen and siRNA-mediated gene knockdown indicated that regeneration requires canonical Wnt signaling. Wnt3a protein rescued both caspase-blocked regeneration and branchial sac growth. Inhibition of apoptosis did not affect branchial sac stem cell proliferation but prevented the survival of progenitor cells. After bisection across the mid-body, apoptosis occurred only in the regenerating basal fragments, although both fragments contained a part of the branchial sac, suggesting that apoptosis is unilateral at the wound site and the presence of branchial sac stem cells is insufficient for regeneration. The results suggest that apoptosis-dependent Wnt signaling mediates regeneration and homeostatic growth in Ciona.

Summary: Apoptosis induces Wnt-dependent regeneration and homeostatic cell renewal in Ciona. Apoptosis is required for stem cell survival and is absent in non-regenerating body parts, suggesting a role in asymmetrical regeneration.

Transcription Factors of the bHLH Family Delineate Vertebrate Landmarks in the Nervous System of a Simple Chordate

Tunicates are marine invertebrates whose tadpole-like larvae feature a highly simplified version of the chordate body plan. Similar to their distant vertebrate relatives, tunicate larvae develop a regionalized central nervous system and form distinct neural structures, which include a rostral sensory vesicle, a motor ganglion, and a caudal nerve cord. The sensory vesicle contains a photoreceptive complex and a statocyst, and based on the comparable expression patterns of evolutionarily conserved marker genes, it is believed to include proto-hypothalamic and proto-retinal territories. The evolutionarily conserved molecular fingerprints of these landmarks of the vertebrate brain consist of genes encoding for different transcription factors, and of the gene batteries that they control, and include several members of the bHLH family. Here we review the complement of bHLH genes present in the streamlined genome of the tunicate Ciona robusta and their current classification, and summarize recent studies on proneural bHLH transcription factors and their expression territories. We discuss the possible roles of bHLH genes in establishing the molecular compartmentalization of the enticing nervous system of this unassuming chordate.

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.

Integrin-alpha-6+ Candidate stem cells are responsible for whole body regeneration in the invertebrate chordate Botrylloides diegensis

Colonial ascidians are the only chordates able to undergo whole body regeneration (WBR), during which entire new bodies can be regenerated from small fragments of blood vessels. Here, we show that during the early stages of WBR in Botrylloides diegensis, proliferation occurs only in small, blood-borne cells that express integrin-alpha-6 (IA6), pou3 and vasa. WBR cannot proceed when proliferating IA6+ cells are ablated with Mitomycin C, and injection of a single IA6+ Candidate stem cell can rescue WBR after ablation. Lineage tracing using EdU-labeling demonstrates that donor-derived IA6+ Candidate stem cells directly give rise to regenerating tissues. Inhibitors of either Notch or canonical Wnt signaling block WBR and reduce proliferation of IA6+ Candidate stem cells, indicating that these two pathways regulate their activation. In conclusion, we show that IA6+ Candidate stem cells are responsible for whole body regeneration and give rise to regenerating tissues.

Clonal ascidians are able to undergo whole body regeneration (WBR), where entire new bodies can be regenerated from blood vessel fragments. Here, the authors provide evidence in Botrylloides diegensis supporting pou3 and vasa expressing blood-borne cells isolated with anti-IA6 antibody as candidate stem cells responsible for WBR.

Progenitor targeting by adult stem cells in Ciona homeostasis, injury, and regeneration

In the ascidian Ciona intestinalis, oral siphon amputation activates adult stem cell niches in the branchial sac to divide and dispatch migratory progenitor cells to a regeneration blastema at the site of injury. This study shows that progenitor cells derived from branchial sac stem cell niches have roles in homeostasis, wound repair, and regeneration of the siphons and neural complex (NC). During homeostasis, progenitor cells targeted the pharyngeal stigmata to replace ciliated cells involved in filter feeding. After individual or double siphon amputations, progenitor cells specifically targeted the oral or atrial siphons or both siphons, and were involved in the replacement of siphon circular muscle fibers. After oral siphon wounding, progenitor cells targeted the wound sites, and in some cases a supernumerary siphon was formed, although progenitor cell targeting did not predict the induction of supernumerary siphons. Following NC ablation, progenitor cells specifically targeted the regenerating NC, and supplied the precursors of new brain and neural gland cells. The tissues and organs targeted by branchial sac stem cells exhibited apoptosis during homeostasis and injury. It is concluded that branchial sac progenitor cells are multipotent and show targeting specificity that is correlated with apoptosis during homeostatic growth, tissue repair, and regeneration.

Cellular and molecular mechanisms of regeneration in colonial and solitary Ascidians

Regenerative ability is highly variable among the metazoans. While many invertebrate organisms are capable of complete regeneration of entire bodies and organs, whole-organ regeneration is limited to very few species in the vertebrate lineages. Tunicates, which are invertebrate chordates and the closest extant relatives of the vertebrates, show robust regenerative ability. Colonial ascidians of the family of the Styelidae, such as several species of Botrylloides, are able to regenerate entire new bodies from nothing but fragments of vasculature, and they are the only chordates that are capable of whole body regeneration. The cell types and signaling pathways involved in whole body regeneration are not well understood, but some evidence suggests that blood borne cells may play a role. Solitary ascidians such as Ciona can regenerate the oral siphon and their central nervous system, and stem cells located in the branchial sac are required for this regeneration. Here, we summarize the cellular and molecular mechanisms of tunicate regeneration that have been identified so far and discuss differences and similarities between these mechanisms in regenerating tunicate species.

Analysis of sea star larval regeneration reveals conserved processes of whole-body regeneration across the metazoa

Background

Metazoan lineages exhibit a wide range of regenerative capabilities that vary among developmental stage and tissue type. The most robust regenerative abilities are apparent in the phyla Cnidaria, Platyhelminthes, and Echinodermata, whose members are capable of whole-body regeneration (WBR). This phenomenon has been well characterized in planarian and hydra models, but the molecular mechanisms of WBR are less established within echinoderms, or any other deuterostome system. Thus, it is not clear to what degree aspects of this regenerative ability are shared among metazoa.

Results

We characterize regeneration in the larval stage of the Bat Star (Patiria miniata). Following bisection along the anterior-posterior axis, larvae progress through phases of wound healing and re-proportioning of larval tissues. The overall number of proliferating cells is reduced following bisection, and we find evidence for a re-deployment of genes with known roles in embryonic axial patterning. Following axial respecification, we observe a significant localization of proliferating cells to the wound region. Analyses of transcriptome data highlight the molecular signatures of functions that are common to regeneration, including specific signaling pathways and cell cycle controls. Notably, we find evidence for temporal similarities among orthologous genes involved in regeneration from published Platyhelminth and Cnidarian regeneration datasets.

Conclusions

These analyses show that sea star larval regeneration includes phases of wound response, axis respecification, and wound-proximal proliferation. Commonalities of the overall process of regeneration, as well as gene usage between this deuterostome and other species with divergent evolutionary origins reveal a deep similarity of whole-body regeneration among the metazoa.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0633-9) contains supplementary material, which is available to authorized users.

Tissue Regeneration without Stem Cell Transplantation: Self-Healing Potential from Ancestral Chemistry and Physical Energies

The human body constantly regenerates after damage due to the self-renewing and differentiating properties of its resident stem cells. To recover the damaged tissues and regenerate functional organs, scientific research in the field of regenerative medicine is firmly trying to understand the molecular mechanisms through which the regenerative potential of stem cells may be unfolded into a clinical application. The finding that some organisms are capable of regenerative processes and the study of conserved evolutionary patterns in tissue regeneration may lead to the identification of natural molecules of ancestral species capable to extend their regenerative potential to human tissues. Such a possibility has also been strongly suggested as a result of the use of physical energies, such as electromagnetic fields and mechanical vibrations in human adult stem cells. Results from scientific studies on stem cell modulation confirm the possibility to afford a chemical manipulation of stem cell fate in vitro and pave the way to the use of natural molecules, as well as electromagnetic fields and mechanical vibrations to target human stem cells in their niche inside the body, enhancing human natural ability for self-healing.

De novo draft assembly of the Botrylloides leachii genome provides further insight into tunicate evolution

Tunicates are marine invertebrates that compose the closest phylogenetic group to the vertebrates. These chordates present a particularly diverse range of regenerative abilities and life-history strategies. Consequently, tunicates provide an extraordinary perspective into the emergence and diversity of these traits. Here we describe the genome sequencing, annotation and analysis of the Stolidobranchian Botrylloides leachii. We have produced a high-quality 159 Mb assembly, 82% of the predicted 194 Mb genome. Analysing genome size, gene number, repetitive elements, orthologs clustering and gene ontology terms show that B. leachii has a genomic architecture similar to that of most solitary tunicates, while other recently sequenced colonial ascidians have undergone genome expansion. In addition, ortholog clustering has identified groups of candidate genes for the study of colonialism and whole-body regeneration. By analysing the structure and composition of conserved gene linkages, we observed examples of cluster breaks and gene dispersions, suggesting that several lineage-specific genome rearrangements occurred during tunicate evolution. We also found lineage-specific gene gain and loss within conserved cell-signalling pathways. Such examples of genetic changes within conserved cell-signalling pathways commonly associated with regeneration and development that may underlie some of the diverse regenerative abilities observed in tunicates. Overall, these results provide a novel resource for the study of tunicates and of colonial ascidians.

Developmental signature, synaptic connectivity and neurotransmission are conserved between vertebrate hair cells and tunicate coronal cells

In tunicates, the coronal organ represents a sentinel checking particle entrance into the pharynx. The organ differentiates from an anterior embryonic area considered a proto-placode. For their embryonic origin, morphological features and function, coronal sensory cells have been hypothesized to be homologues to vertebrate hair cells. However, vertebrate hair cells derive from a posterior placode. This contradicts one of the principle historical criteria for homology, similarity of position, which could be taken as evidence against coronal cells/hair cells homology. In the tunicates Ciona intestinalis and C. robusta, we found that the coronal organ expresses genes (Atoh, Notch, Delta-like, Hairy-b, and Musashi) characterizing vertebrate neural and hair cell development. Moreover, coronal cells exhibit a complex synaptic connectivity pattern, and express neurotransmitters (Glu, ACh, GABA, 5-HT, and catecholamines), or enzymes for their synthetic machinery, involved in hair cell activity. Lastly, coronal cells express the Trpa gene, which encodes an ion channel expressed in hair cells. These data lead us to hypothesize a model in which competence to make secondary mechanoreceptors was initially broadly distributed through placode territories, but has become confined to different placodes during the evolution of the vertebrate and tunicate lineages.

Solitary Ascidians as Model Organisms in Regenerative Biology Studies

Regeneration, the process of replacing lost or damaged body parts, has long captured human imagination and is a key feature among all animal phyla. Due to their close phylogenetic relationship to vertebrates and their high regenerative abilities, ascidians (Chordata, Ascidiacea) are often used as models to shed light on the cellular and genetic process involved in tissue regeneration. Surprisingly, ascidian regeneration studies are based on only a few model species. In this chapter, we point out the important potential of solitary ascidians in regenerative and stem cell studies. We review recent studies of regeneration among solitary ascidians and discuss the cellular mechanism of tissue regeneration and the possible involvement of circulating cells in these processes. New data regarding the relationship between age and regeneration abilities of the solitary ascidian Polycarpa mytiligera (Stolidobranchia, Styelidae) are presented. The unique regeneration abilities found in P. mytiligera following evisceration of its digestive system and following amputation of its neural complex and siphon-associated structures and nerves imply on its potential to serve as a novel model system for understanding tissue regeneration.

A microRNA-mRNA expression network during oral siphon regeneration in Ciona

Here we present a parallel study of mRNA and microRNA expression during oral siphon (OS) regeneration in Ciona robusta, and the derived network of their interactions. In the process of identifying 248 mRNAs and 15 microRNAs as differentially expressed, we also identified 57 novel microRNAs, several of which are among the most highly differentially expressed. Analysis of functional categories identified enriched transcripts related to stress responses and apoptosis at the wound healing stage, signaling pathways including Wnt and TGFβ during early regrowth, and negative regulation of extracellular proteases in late stage regeneration. Consistent with the expression results, we found that inhibition of TGFβ signaling blocked OS regeneration. A correlation network was subsequently inferred for all predicted microRNA-mRNA target pairs expressed during regeneration. Network-based clustering associated transcripts into 22 non-overlapping groups, the functional analysis of which showed enrichment of stress response, signaling pathway and extracellular protease categories that could be related to specific microRNAs. Predicted targets of the miR-9 cluster suggest a role in regulating differentiation and the proliferative state of neural progenitors through regulation of the cytoskeleton and cell cycle.

Ciona as a Simple Chordate Model for Heart Development and Regeneration

Cardiac cell specification and the genetic determinants that govern this process are highly conserved among Chordates. Recent studies have established the importance of evolutionarily-conserved mechanisms in the study of congenital heart defects and disease, as well as cardiac regeneration. As a basal Chordate, the Ciona model system presents a simple scaffold that recapitulates the basic blueprint of cardiac development in Chordates. Here we will focus on the development and cellular structure of the heart of the ascidian Ciona as compared to other Chordates, principally vertebrates. Comparison of the Ciona model system to heart development in other Chordates presents great potential for dissecting the genetic mechanisms that underlie congenital heart defects and disease at the cellular level and might provide additional insight into potential pathways for therapeutic cardiac regeneration.

ANISEED 2015: a digital framework for the comparative developmental biology of ascidians

Ascidians belong to the tunicates, the sister group of vertebrates and are recognized model organisms in the field of embryonic development, regeneration and stem cells. ANISEED is the main information system in the field of ascidian developmental biology. This article reports the development of the system since its initial publication in 2010. Over the past five years, we refactored the system from an initial custom schema to an extended version of the Chado schema and redesigned all user and back end interfaces. This new architecture was used to improve and enrich the description of Ciona intestinalis embryonic development, based on an improved genome assembly and gene model set, refined functional gene annotation, and anatomical ontologies, and a new collection of full ORF cDNAs. The genomes of nine ascidian species have been sequenced since the release of the C. intestinalis genome. In ANISEED 2015, all nine new ascidian species can be explored via dedicated genome browsers, and searched by Blast. In addition, ANISEED provides full functional gene annotation, anatomical ontologies and some gene expression data for the six species with highest quality genomes. ANISEED is publicly available at: http://www.aniseed.cnrs.fr.