Early segregation of germ and somatic lineages during gonadal regeneration in the annelid Enchytraeus japonensis

Annelids as models of germ cell and gonad regeneration

Germ cells (reproductive cells and their progenitors) give rise to the next generation in sexually reproducing organisms. The loss or removal of germ cells often leads to sterility in established research organisms such as the fruit fly, nematodes, frog, and mouse. The failure to regenerate germ cells in these organisms reinforced the dogma of germline-soma barrier in which germ cells are set-aside during embryogenesis and cannot be replaced by somatic cells. However, in stark contrast, many animals including segmented worms (annelids), hydrozoans, planaria, sea stars, sea urchins, and tunicates can regenerate germ cells. Here I review germ cell and gonad regeneration in annelids, a rich history of research that dates back to the early 20th century in this highly regenerative group. Examples include annelids from across the annelid phylogeny, across developmental stages, and reproductive strategies. Adult annelids regenerate germ cells as a part of regeneration, grafting, and asexual reproduction. Annelids can also recover germ cells after ablation of germ cell progenitors in the embryos. I present a framework to investigate cellular sources of germ cell regeneration in annelids, and discuss the literature that supports different possibilities within this framework, where germ-soma separation may or may not be preserved. With contemporary genetic-lineage tracing and bioinformatics tools, and several genetically enabled annelid models, we are at the brink of answering the big questions that puzzled many for over more than a century.

Annelid adult cell type diversity and their pluripotent cellular origins

Many annelids can regenerate missing body parts or reproduce asexually, generating all cell types in adult stages. However, the putative adult stem cell populations involved in these processes, and the diversity of cell types generated by them, are still unknown. To address this, we recover 75,218 single cell transcriptomes of the highly regenerative and asexually-reproducing annelid Pristina leidyi. Our results uncover a rich cell type diversity including annelid specific types as well as novel types. Moreover, we characterise transcription factors and gene networks that are expressed specifically in these populations. Finally, we uncover a broadly abundant cluster of putative stem cells with a pluripotent signature. This population expresses well-known stem cell markers such as vasa, piwi and nanos homologues, but also shows heterogeneous expression of differentiated cell markers and their transcription factors. We find conserved expression of pluripotency regulators, including multiple chromatin remodelling and epigenetic factors, in piwi+ cells. Finally, lineage reconstruction analyses reveal computational differentiation trajectories from piwi+ cells to diverse adult types. Our data reveal the cell type diversity of adult annelids by single cell transcriptomics and suggest that a piwi+ cell population with a pluripotent stem cell signature is associated with adult cell type differentiation.

Germ Line/Multipotency Genes Show Differential Expression during Embryonic Development of the Annelid Enchytraeus coronatus

Germ line development and the origin of the primordial germ cells (PGCs) are very variable and may occur across a range of developmental stages and in several developmental contexts. In establishing and maintaining germ line, a conserved set of genes is involved. On the other hand, these genes are expressed in multipotent/pluripotent cells that may give rise to both somatic and germline cells. To begin elucidating mechanisms by which the germ line is specified in Enchytraeus coronatus embryos, we identified twenty germline/multipotency genes, homologs of Vasa, PL10, Piwi, Nanos, Myc, Pumilio, Tudor, Boule, and Bruno, using transcriptome analysis and gene cloning, and characterized their expression by whole-mount in situ hybridization. To answer the question of the possible origin of PGCs in this annelid, we carried out an additional description of the early embryogenesis. Our results suggest that PGCs derive from small cells originating at the first two divisions of the mesoteloblasts. PGCs form two cell clusters, undergo limited proliferation, and migrate to the developing gonadal segments. In embryos and juvenile E. coronatus, homologs of the germline/multipotency genes are differentially expressed in both germline and somatic tissue including the presumptive germ cell precursors, posterior growth zone, developing foregut, and nervous system.

Vasa, Piwi, and Pl10 Expression during Sexual Maturation and Asexual Reproduction in the Annelid Pristina longiseta

Naidids are tiny, transparent freshwater oligochaetes, which are well known for their ability to propagate asexually. Despite the fact that sexually mature individuals and cocoons with embryos are sometimes found in nature, in long-period laboratory cultures, worms reproduce agametically only. In this paper, we showed, for the first time, the expression of Vasa, Piwi, and Pl10 homologs in mature Pristina longiseta worms with well-developed reproductive system structures and germ cells. Although the animals have been propagated asexually by paratomic fission for over 20 years in our lab, some individuals become sexualized under standard conditions for our laboratory culture and demonstrate various stages of maturation. The fully matured animals developed a complete set of sexual apparatus including spermatheca, atrium, seminal vesicles, and ovisac. They also had a clitellum and were able to form cocoons. The cues for the initiation of sexual maturation are still unknown for P. longiseta; nevertheless, our data suggest that the laboratory strain of P. longiseta maintains the ability to become fully sexually mature and to establish germline products even after a long period of agametic reproduction. On the other hand, many of the sexualized worms formed a fission zone and continued to reproduce asexually. Thus, in this species, the processes of asexual reproduction and sexual maturation do not preclude each other, and Vasa, Piwi, and Pl10 homologs are expressed in both somatic and germline tissue including the posterior growth zone, fission zone, nervous system, germline cells, and gametes.

Cell Proliferation Indices in Regenerating Alitta virens (Annelida, Errantia)

In recent years, interest in the possible molecular regulators of cell proliferation and differentiation in a wide range of regeneration models has grown significantly, but the cell kinetics of this process remain largely a mystery. Here we try to elucidate the cellular aspects of regeneration by EdU incorporation in intact and posteriorly amputated annelid Alitta virens using quantitative analysis. We found that the main mechanism of blastema formation in A. virens is local dedifferentiation; mitotically active cells of intact segments do not significantly contribute to the blastemal cellular sources. Amputation-induced proliferation occurred predominantly within the epidermal and intestinal epithelium, as well as wound-adjacent muscle fibers, where clusters of cells at the same stage of the cell cycle were found. The resulting regenerative bud had zones of high proliferative activity and consisted of a heterogeneous population of cells that differed in their anterior-posterior positions and in their cell cycle parameters. The data presented allowed for the quantification of cell proliferation in the context of annelid regeneration for the first time. Regenerative cells showed an unprecedentedly high cycle rate and an exceptionally large growth fraction, making this regeneration model especially valuable for studying coordinated cell cycle entry in vivo in response to injury.

Distinct patterns of gene expression during regeneration and asexual reproduction in the annelid Pristina leidyi

Regeneration, the ability to replace lost body parts, is a widespread phenomenon in the animal kingdom often connected to asexual reproduction or fission, since the only difference between the two appears to be the stimulus that triggers them. Both developmental processes have largely been characterized; however, the molecular toolkit and genetic mechanisms underlying these events remain poorly unexplored. Annelids, in particular the oligochaete Pristina leidyi, provide a good model system to investigate these processes as they show diverse ways to regenerate, and can reproduce asexually through fission under laboratory conditions. Here, we used a comparative transcriptomics approach based on RNA-sequencing and differential gene expression analyses to understand the molecular mechanisms involved in anterior regeneration and asexual reproduction. We found 291 genes upregulated during anterior regeneration, including several regeneration-related genes previously reported in other annelids such as frizzled, paics, and vdra. On the other hand, during asexual reproduction, 130 genes were found upregulated, and unexpectedly, many of them were related to germline development during sexual reproduction. We also found important differences between anterior regeneration and asexual reproduction, with the latter showing a gene expression profile more similar to that of control individuals. Nevertheless, we identified 35 genes that were upregulated in both conditions, many of them related to cell pluripotency, stem cells, and cell proliferation. Overall, our results shed light on the molecular mechanisms that control anterior regeneration and asexual reproduction in annelids and reveal similarities with other animals, suggesting that the genetic machinery controlling these processes is conserved across metazoans.

Multicellularity in animals: The potential for within-organism conflict

Metazoans function as individual organisms but also as “colonies” of cells whose single-celled ancestors lived and reproduced independently. Insights from evolutionary biology about multicellular group formation help us understand the behavior of cells: why they cooperate, and why cooperation sometimes breaks down. Current explanations for multicellularity focus on two aspects of development which promote cooperation and limit conflict among cells: a single-cell bottleneck, which creates organisms composed of clones, and a separation of somatic and germ cell lineages, which reduces the selective advantage of cheating. However, many obligately multicellular organisms thrive with neither, creating the potential for within-organism conflict. Here, we argue that the prevalence of such organisms throughout the Metazoa requires us to refine our preconceptions of conflict-free multicellularity. Evolutionary theory must incorporate developmental mechanisms across a broad range of organisms—such as unusual reproductive strategies, totipotency, and cell competition—while developmental biology must incorporate evolutionary principles. To facilitate this cross-disciplinary approach, we provide a conceptual overview from evolutionary biology for developmental biologists, using analogous examples in the well-studied social insects.

Sex-Inducing Activities of the Land Planarian Bipalium nobile Extract Fractions, Obtained Using Bioassay-Guided Fractionation, in the Freshwater Planarian Dugesia ryukyuensis

Sexually mature planarians produce sex-inducing substances that induce postembryonic development of hermaphroditic reproductive organs in asexual freshwater planarians. Although the sex-inducing substances may be useful for elucidating the mechanism underlying this reproductive switch, the available information is limited. The potency of sex-inducing activity is conserved, at least at the order level. Recently, we showed that the sex-inducing activity in the land planarian Bipalium nobile was much higher than that in freshwater planarians. In the present study, we performed bioassay-guided fractionation of the sex-inducing substances produced by B. nobile and propose that crucial sex-inducing activity that triggers complete sexualization for asexual worms of the freshwater planarian Dugesia ryukyuensis is produced by additive and/or synergetic effects of various sex-inducing substances involved in ovarian development. The current study provided an isolation scheme for the minimum-required combination of sex-inducing substances for producing crucial sex-inducing activity.

Regeneration in the Segmented Annelid Capitella teleta

The segmented worms, or annelids, are a clade within the Lophotrochozoa, one of the three bilaterian superclades. Annelids have long been models for regeneration studies due to their impressive regenerative abilities. Furthermore, the group exhibits variation in adult regeneration abilities with some species able to replace anterior segments, posterior segments, both or neither. Successful regeneration includes regrowth of complex organ systems, including the centralized nervous system, gut, musculature, nephridia and gonads. Here, regenerative capabilities of the annelid Capitella teleta are reviewed. C. teleta exhibits robust posterior regeneration and benefits from having an available sequenced genome and functional genomic tools available to study the molecular and cellular control of the regeneration response. The highly stereotypic developmental program of C. teleta provides opportunities to study adult regeneration and generate robust comparisons between development and regeneration.

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.

Comparative Aspects of Annelid Regeneration: Towards Understanding the Mechanisms of Regeneration

The question of why animals vary in their ability to regenerate remains one of the most intriguing questions in biology. Annelids are a large and diverse phylum, many members of which are capable of extensive regeneration such as regrowth of a complete head or tail and whole-body regeneration, even from few segments. On the other hand, some representatives of both of the two major annelid clades show very limited tissue regeneration and are completely incapable of segmental regeneration. Here we review experimental and descriptive data on annelid regeneration, obtained at different levels of organization, from data on organs and tissues to intracellular and transcriptomic data. Understanding the variety of the cellular and molecular basis of regeneration in annelids can help one to address important questions about the role of stem/dedifferentiated cells and "molecular morphallaxis" in annelid regeneration as well as the evolution of regeneration in general.

The Germline Marker Piwi Expressed in the Skin Layer of the Polychaete Perinereis wilsoni After Injury

Nereidid polychaete Perinereis wilsoni is a homonomous metameric worm with a complete septum between each segment. Each segment has germ cells localized in the distal area of the parapodia. Perinereis wilsoni is also known to have high abilities of tissue regeneration; however, it is still unclear whether germ cells can regenerate in the healing tissue. To address this, we surgically operated the parapodia of an adult worm to remove germ cells from the segments and observed the germ cell regeneration using the germ cell genetic marker Pw-piwi. At day 20 post-surgical operation of the parapodia in one side of the segment, we found that Pw-piwi was expressed in the regenerating parapodia. We surgically operated the parapodia on both sides of the segment to remove the germ cells completely and it gave a similar result. However, before the expression of this gene marker in the regenerating parapodia, we observed that Pw-piwi was expressed in cells in the skin layer of the worm just after surgical operations. These Pw-piwi-positive cells were not observed in the un-operated worm. Our observations showed that germ cells of Perinereis wilsoni can regenerate even after the complete removal of germ cells from the defined habitat. The Pw-piwipositive cells that appeared in the skin layer after the disappearance of germ cells may be involved in the regeneration of new germ cells.

Injury-Induced Innate Immune Response During Segment Regeneration of the Earthworm, Eisenia andrei

Regeneration of body parts and their interaction with the immune response is a poorly understood aspect of earthworm biology. Consequently, we aimed to study the mechanisms of innate immunity during regeneration in Eisenia andrei earthworms. In the course of anterior and posterior regeneration, we documented the kinetical aspects of segment restoration by histochemistry. Cell proliferation peaked at two weeks and remitted by four weeks in regenerating earthworms. Apoptotic cells were present throughout the cell renewal period. Distinct immune cell (e.g., coelomocyte) subsets were accumulated in the newly-formed blastema in the close proximity of the apoptotic area. Regenerating earthworms have decreased pattern recognition receptors (PRRs) (e.g., TLR, except for scavenger receptor) and antimicrobial peptides (AMPs) (e.g., lysenin) mRNA patterns compared to intact earthworms. In contrast, at the protein level, mirroring regulation of lysenins became evident. Experimental coelomocyte depletion caused significantly impaired cell divisions and blastema formation during anterior and posterior regeneration. These obtained novel data allow us to gain insight into the intricate interactions of regeneration and invertebrate innate immunity.

Quantifying Cell Proliferation During Regeneration of Aquatic Worms

Many species of aquatic worms, including members of the phyla Nemertea, Annelida, Platyhelminthes, and Xenacoelomorpha, can regenerate large parts of their body after amputation. In most species, cell proliferation plays key roles in the reconstruction of lost tissues. For example, in annelids and flatworms, inhibition of cell proliferation by irradiation or chemicals prevents regeneration. Cell proliferation also plays crucial roles in growth, body patterning (e.g., segmentation) and asexual reproduction in many groups of aquatic worms. Cell proliferation dynamics in these organisms can be studied using immunohistochemical detection of proteins expressed during proliferation-associated processes or by incorporation and labeling of thymidine analogues during DNA replication. In this chapter, we present protocols for labeling and quantifying cell proliferation by (a) antibody-based detection of either phosphorylated histone H3 during mitosis or proliferating cell nuclear antigen (PCNA) during S-phase, and (b) incorporation of two thymidine analogues, 5'-bromo-2'-deoxyuridine (BrdU) and 5'-ethynyl-2'-deoxyuridine (EdU), detected by immunohistochemistry or inorganic "click" chemistry, respectively. Although these protocols have been developed for whole mounts of small (<2 cm) marine and freshwater worms, they can also be adapted for use in larger specimens or tissue sections.

Comparative transcriptomics in Syllidae (Annelida) indicates that posterior regeneration and regular growth are comparable, while anterior regeneration is a distinct process

Background

Annelids exhibit remarkable postembryonic developmental abilities. Most annelids grow during their whole life by adding segments through the action of a segment addition zone (SAZ) located in front of the pygidium. In addition, they show an outstanding ability to regenerate their bodies. Experimental evidence and field observations show that many annelids are able to regenerate their posterior bodies, while anterior regeneration is often limited or absent. Syllidae, for instance, usually show high abilities of posterior regeneration, although anterior regeneration varies across species. Some syllids are able to partially restore the anterior end, while others regenerate all lost anterior body after bisection. Here, we used comparative transcriptomics to detect changes in the gene expression profiles during anterior regeneration, posterior regeneration and regular growth of two syllid species: Sphaerosyllis hystrix and Syllis gracilis; which exhibit limited and complete anterior regeneration, respectively.

Results

We detected a high number of genes with differential expression: 4771 genes in S. hystrix (limited anterior regeneration) and 1997 genes in S. gracilis (complete anterior regeneration). For both species, the comparative transcriptomic analysis showed that gene expression during posterior regeneration and regular growth was very similar, whereas anterior regeneration was characterized by up-regulation of several genes. Among the up-regulated genes, we identified putative homologs of regeneration-related genes associated to cellular proliferation, nervous system development, establishment of body axis, and stem-cellness; such as rup and JNK (in S. hystrix); and glutamine synthetase, elav, slit, Hox genes, β-catenin and PL10 (in S. gracilis).

Conclusions

Posterior regeneration and regular growth show no significant differences in gene expression in the herein investigated syllids. However, anterior regeneration is associated with a clear change in terms of gene expression in both species. Our comparative transcriptomic analysis was able to detect differential expression of some regeneration-related genes, suggesting that syllids share some features of the regenerative mechanisms already known for other annelids and invertebrates.

Morphological, cellular and molecular characterization of posterior regeneration in the marine annelid Platynereis dumerilii

Regeneration, the ability to restore body parts after an injury or an amputation, is a widespread but highly variable and complex phenomenon in animals. While having fascinated scientists for centuries, fundamental questions about the cellular basis of animal regeneration as well as its evolutionary history remain largely unanswered. Here, we present a study of regeneration of the marine annelid Platynereis dumerilii, an emerging comparative developmental biology model, which, like many other annelids, displays important regenerative abilities. When P. dumerilii worms are amputated, they are able to regenerate the posteriormost differentiated part of their body and a stem cell-rich growth zone that allows the production of new segments replacing the amputated ones. We show that posterior regeneration is a rapid process that follows a well reproducible path and timeline, going through specific stages that we thoroughly defined. Wound healing is achieved one day after amputation and a regeneration blastema forms one day later. At this time point, some tissue specification already occurs, and a functional posterior growth zone is re-established as early as three days after amputation. Regeneration timing is only influenced, in a minor manner, by worm size. Comparable regenerative abilities are found for amputations performed at different positions along the antero-posterior axis of the worm, except when amputation planes are very close to the pharynx. Regenerative abilities persist upon repeated amputations without important alterations of the process. We also show that intense cell proliferation occurs during regeneration and that cell divisions are required for regeneration to proceed normally. Finally, 5-ethynyl-2'-deoxyuridine (EdU) pulse and chase experiments suggest that blastemal cells mostly derive from the segment immediately abutting the amputation plane. The detailed characterization of P. dumerilii posterior body regeneration presented in this article provides the foundation for future mechanistic and comparative studies of regeneration in this species.

EvoRegen in animals: Time to uncover deep conservation or convergence of adult stem cell evolution and regenerative processes

How do animals regenerate specialised tissues or their entire body after a traumatic injury, how has this ability evolved and what are the genetic and cellular components underpinning this remarkable feat? While some progress has been made in understanding mechanisms, relatively little is known about the evolution of regenerative ability. Which elements of regeneration are due to lineage specific evolutionary novelties or have deeply conserved roots within the Metazoa remains an open question. The renaissance in regeneration research, fuelled by the development of modern functional and comparative genomics, now enable us to gain a detailed understanding of both the mechanisms and evolutionary forces underpinning regeneration in diverse animal phyla. Here we review existing and emerging model systems, with the focus on invertebrates, for studying regeneration. We summarize findings across these taxa that tell us something about the evolution of adult stem cell types that fuel regeneration and the growing evidence that many highly regenerative animals harbor adult stem cells with a gene expression profile that overlaps with germline stem cells. We propose a framework in which regenerative ability broadly evolves through changes in the extent to which stem cells generated through embryogenesis are maintained into the adult life history.

Investigation into the cellular origins of posterior regeneration in the annelid Capitella teleta

Many animals can regenerate, although there is great diversity in regenerative capabilities. A major question in regenerative biology is determining the cellular source of newly formed tissue. The polychaete annelid, Capitella teleta, can regenerate posterior segments following transverse amputation. However, the source, behavior and molecular characteristics of the cells that form new tissue during regeneration are largely unknown. Using an indirect cell tracking method involving 5′‐ethynyl‐2′‐deoxyuridine (EdU) incorporation, we show that cell migration occurs during C. teleta posterior regeneration. Expression of the multipotency/germ line marker CapI‐vasa led us to hypothesize that stem cells originate from a multipotent progenitor cell (MPC) cluster, migrate through the coelomic cavity, and contribute to regeneration of tissue. We show that the capacity for posterior regeneration and segment formation is greater with than without the MPC cluster. Finally, we propose a working model of posterior regeneration in C. teleta. This work is the first in C. teleta that addresses the potential source of cells contributing to posterior regeneration, and may provide clues as to why some animals are highly successful regenerators.

The identification of ᴅ-tryptophan as a bioactive substance for postembryonic ovarian development in the planarian Dugesia ryukyuensis

Many metazoans start germ cell development during embryogenesis, while some metazoans possessing pluripotent stem cells undergo postembryonic germ cell development. The latter reproduce asexually but develop germ cells from pluripotent stem cells or dormant primordial germ cells when they reproduce sexually. Sexual induction of the planarian Dugesia ryukyuensis is an important model for postembryonic germ cell development. In this experimental system, hermaphroditic reproductive organs are differentiated in presumptive gonadal regions by the administration of a crude extract from sexual planarians to asexual ones. However, the substances involved in the first event during postembryonic germ cell development, i.e., ovarian development, remain unknown. Here, we aimed to identify a bioactive compound associated with postembryonic ovarian development. Bioassay-guided fractionation identified ʟ-tryptophan (Trp) on the basis of electrospray ionization–mass spectrometry, circular dichroism, and nuclear magnetic resonance spectroscopy. Originally masked by a large amount of ʟ-Trp, ᴅ-Trp was detected by reverse-phase high-performance liquid chromatography. The ovary-inducing activity of ᴅ-Trp was 500 times more potent than that of ʟ-Trp. This is the first report describing a role for an intrinsic ᴅ-amino acid in postembryonic germ cell development. Our findings provide a novel insight into the mechanisms of germ cell development regulated by low-molecular weight bioactive compounds.

Plasticity and regeneration of gonads in the annelid Pristina leidyi

Background

Gonads are specialized gamete-producing structures that, despite their functional importance, are generated by diverse mechanisms across groups of animals and can be among the most plastic organs of the body. Annelids, the segmented worms, are a group in which gonads have been documented to be plastic and to be able to regenerate, but little is known about what factors influence gonad development or how these structures regenerate. In this study, we aimed to identify factors that influence the presence and size of gonads and to investigate gonad regeneration in the small asexually reproducing annelid, Pristina leidyi.

Results

We found that gonad presence and size in asexual adult P. leidyi are highly variable across individuals and identified several factors that influence these structures. An extrinsic factor, food availability, and two intrinsic factors, individual age and parental age, strongly influence the presence and size of gonads in P. leidyi. We also found that following head amputation in this species, gonads can develop by morphallactic regeneration in previously non-gonadal segments. We also identified a sexually mature individual from our laboratory culture that demonstrates that, although our laboratory strain reproduces only asexually, it retains the potential to become fully sexual.

Conclusions

Our findings demonstrate that gonads in P. leidyi display high phenotypic plasticity and flexibility with respect to their presence, their size, and the segments in which they can form. Considering our findings along with relevant data from other species, we find that, as a group, clitellate annelids can form gonads in at least four different contexts: post-starvation refeeding, fission, morphallactic regeneration, and epimorphic regeneration. This group is thus particularly useful for investigating the mechanisms involved in gonad formation and the evolution of post-embryonic phenotypic plasticity.

Electronic supplementary material

The online version of this article (doi:10.1186/s13227-016-0059-1) contains supplementary material, which is available to authorized users.

Developmental and molecular biology of annelid regeneration: a comparative review of recent studies

Studies of annelid regeneration have greatly increased in frequency in recent years, providing new insights into the developmental basis and evolution of regeneration. In this review, we summarize recent findings related to regeneration in annelids, focusing on molecular and developmental studies of epimorphic (blastema-based) regeneration, morphallactic (tissue-remodeling based) regeneration, and development and regeneration of putative stem cells of the posterior growth zone and germline. Regeneration is being investigated in a broad range of annelids spanning the phylum, and comparing findings among species reveals both widely conserved features that may be ancestral for the phylum as well as features that are variable across the group.

Long-term time-lapse live imaging reveals extensive cell migration during annelid regeneration

BACKGROUND

Time-lapse imaging has proven highly valuable for studying development, yielding data of much finer resolution than traditional "still-shot" studies and allowing direct examination of tissue and cell dynamics. A major challenge for time-lapse imaging of animals is keeping specimens immobile yet healthy for extended periods of time. Although this is often feasible for embryos, the difficulty of immobilizing typically motile juvenile and adult stages remains a persistent obstacle to time-lapse imaging of post-embryonic development.

RESULTS

Here we describe a new method for long-duration time-lapse imaging of adults of the small freshwater annelid Pristina leidyi and use this method to investigate its regenerative processes. Specimens are immobilized with tetrodotoxin, resulting in irreversible paralysis yet apparently normal regeneration, and mounted in agarose surrounded by culture water or halocarbon oil, to prevent dehydration but allowing gas exchange. Using this method, worms can be imaged continuously and at high spatial-temporal resolution for up to 5 days, spanning the entire regeneration process. We performed a fine-scale analysis of regeneration growth rate and characterized cell migration dynamics during early regeneration. Our studies reveal the migration of several putative cell types, including one strongly resembling published descriptions of annelid neoblasts, a cell type suggested to be migratory based on "still-shot" studies and long hypothesized to be linked to regenerative success in annelids.

CONCLUSIONS

Combining neurotoxin-based paralysis, live mounting techniques and a starvation-tolerant study system has allowed us to obtain the most extensive high-resolution longitudinal recordings of full anterior and posterior regeneration in an invertebrate, and to detect and characterize several cell types undergoing extensive migration during this process. We expect the tetrodotoxin paralysis and time-lapse imaging methods presented here to be broadly useful in studying other animals and of particular value for studying post-embryonic development.

A Stable Thoracic Hox Code and Epimorphosis Characterize Posterior Regeneration in Capitella teleta

Regeneration, the ability to replace lost tissues and body parts following traumatic injury, occurs widely throughout the animal tree of life. Regeneration occurs either by remodeling of pre-existing tissues, through addition of new cells by cell division, or a combination of both. We describe a staging system for posterior regeneration in the annelid, Capitella teleta, and use the C. teleta Hox gene code as markers of regional identity for regenerating tissue along the anterior-posterior axis. Following amputation of different posterior regions of the animal, a blastema forms and by two days, proliferating cells are detected by EdU incorporation, demonstrating that epimorphosis occurs during posterior regeneration of C. teleta. Neurites rapidly extend into the blastema, and gradually become organized into discrete nerves before new ganglia appear approximately seven days after amputation. In situ hybridization shows that seven of the ten Hox genes examined are expressed in the blastema, suggesting roles in patterning the newly forming tissue, although neither spatial nor temporal co-linearity was detected. We hypothesized that following amputation, Hox gene expression in pre-existing segments would be re-organized to scale, and the remaining fragment would express the complete suite of Hox genes. Surprisingly, most Hox genes display stable expression patterns in the ganglia of pre-existing tissue following amputation at multiple axial positions, indicating general stability of segmental identity. However, the three Hox genes, CapI-lox4, CapI-lox2 and CapI-Post2, each shift its anterior expression boundary by one segment, and each shift includes a subset of cells in the ganglia. This expression shift depends upon the axial position of the amputation. In C. teleta, thoracic segments exhibit stable positional identity with limited morphallaxis, in contrast with the extensive body remodeling that occurs during regeneration of some other annelids, planarians and acoel flatworms.

Gonad establishment during asexual reproduction in the annelid Pristina leidyi

Animals that can reproduce by both asexual agametic reproduction and sexual reproduction must transmit or re-establish their germ line post-embryonically. Although such a dual reproductive mode has evolved repeatedly among animals, how asexually produced individuals establish their germ line remains poorly understood in most groups. We investigated germ line development in the annelid Pristina leidyi, a species that typically reproduces asexually by paratomic fission, intercalating a new tail and head in the middle of the body followed by splitting. We found that in fissioning individuals, gonads occur in anterior segments in the anterior-most individual as well as in new heads forming within fission zones. Homologs of the germ line/multipotency genes piwi, vasa, and nanos are expressed in the gonads, as well as in proliferative tissues including the posterior growth zone, fission zone, and regeneration blastema. In fissioning animals, certain cells on the ventral nerve cord express a homolog of piwi, are abundant near fission zones, and sometimes make contact with gonads. Such cells are typically undetectable near the blastema and posterior growth zone. Time-lapse imaging provides direct evidence that cells on the ventral nerve cord migrate preferentially towards fission zones. Our findings indicate that gonads form routinely in fissioning individuals, that a population of piwi-positive cells on the ventral nerve cord is associated with fission and gonads, and that cells resembling these piwi-positive cells migrate along the ventral nerve cord. We suggest that the piwi-positive ventral cells are germ cells that transmit the germ line across asexually produced individuals via migration along the ventral nerve cord.

Early events in annelid regeneration: a cellular perspective

The ability to regenerate extensive portions of the body is widespread among the phylum Annelida and this group includes some of the most highly regenerative animals known. Knowledge of the cellular and molecular basis of regeneration in this group is thus important for understanding how regenerative processes have evolved both within the group and across animal phyla. Here, the cellular basis of annelid regeneration is reviewed, with a focus on the earliest steps of regeneration, namely wound-healing and formation of the blastema. Information from a wide range of annelids is compiled in order to identify common and variable elements. There is a large body of valuable older literature on the cellular basis of regeneration in annelids and an effort is made to review this literature in addition to more recent studies. Annelids typically seal the wound through muscular contraction and undergo some autolysis of tissue at the site of the wound. Bodily injury elicits extensive cell migration toward the wound, involving several different types of cells. Some migrating cells form a tissue-clot and phagocytize damaged tissues, whereas others are inferred to contribute to regenerated tissue, specifically mesodermal tissue. In one annelid subgroup, the clitellates, a group of mesodermal cells, sometimes referred to as neoblasts, is inferred to migrate over considerable distances, with cells moving to the wound from several segments away. Epidermis and gut epithelia severed upon amputation typically heal by fusing with like tissue, although not always. After amputation, cellular contacts with the extracellular matrix are disrupted and major changes in cell morphology and adhesion occur within tissues near the wound. Interactions of tissues at the wound appear key for initiating a blastema, with a particularly important role suggested for the ventral nerve cord, although species are variable in this regard; longer-distance effects mediated by the brain are also reported. The anterior-posterior polarity of the blastema can be mis-assigned, leading most commonly to double-headed worms, and the dorsal-ventral polarity of the blastema appears to be induced by the ventral nerve cord. The blastema is thought to arise from contributions of all three tissue layers, with each layer replacing itself in a tissue-specific manner. Blastemal cells originate mostly locally, although some long-distance migration of source-cells is suggested in clitellates. A number of important questions remain about the cellular basis of regeneration in annelids and addressing many of these would be greatly aided by developing approaches to identify and isolate specific cell types and techniques to image and trace cells in vivo.

Evolutionary crossroads in developmental biology: annelids

Annelids (the segmented worms) have a long history in studies of animal developmental biology, particularly with regards to their cleavage patterns during early development and their neurobiology. With the relatively recent reorganisation of the phylogeny of the animal kingdom, and the distinction of the super-phyla Ecdysozoa and Lophotrochozoa, an extra stimulus for studying this phylum has arisen. As one of the major phyla within Lophotrochozoa, Annelida are playing an important role in deducing the developmental biology of the last common ancestor of the protostomes and deuterostomes, an animal from which &gt;98% of all described animal species evolved.

What role do annelid neoblasts play? A comparison of the regeneration patterns in a neoblast-bearing and a neoblast-lacking enchytraeid oligochaete

The term ‘neoblast’ was originally coined for a particular type of cell that had been observed during annelid regeneration, but is now used to describe the pluripotent/totipotent stem cells that are indispensable for planarian regeneration. Despite having the same name, however, planarian and annelid neoblasts are morphologically and functionally distinct, and many annelid species that lack neoblasts can nonetheless substantially regenerate. To further elucidate the functions of the annelid neoblasts, a comparison was made between the regeneration patterns of two enchytraeid oligochaetes, Enchytraeus japonensis and Enchytraeus buchholzi, which possess and lack neoblasts, respectively. In E. japonensis, which can reproduce asexually by fragmentation and subsequent regeneration, neoblasts are present in all segments except for the eight anterior-most segments including the seven head-specific segments, and all body fragments containing neoblasts can regenerate a complete head and a complete tail, irrespective of the region of the body from which they were originally derived. In E. japonensis, therefore, no antero-posterior gradient of regeneration ability exists in the trunk region. However, when amputation was carried out within the head region, where neoblasts are absent, the number of regenerated segments was found to be dependent on the level of amputation along the body axis. In E. buchholzi, which reproduces only sexually and lacks neoblasts in all segments, complete heads were never regenerated and incomplete (hypomeric) heads could be regenerated only from the anterior region of the body. Such an antero-posterior gradient of regeneration ability was observed for both the anterior and posterior regeneration in the whole body of E. buchholzi. These results indicate that the presence of neoblasts correlates with the absence of an antero-posterior gradient of regeneration ability along the body axis, and suggest that the annelid neoblasts are more essential for efficient asexual reproduction than for the regeneration of missing body parts.

Stem cells in asexual reproduction of Enchytraeus japonensis (Oligochaeta, Annelid): proliferation and migration of neoblasts

Enchytraeus japonensis is a small oligochaete that reproduces mainly asexually by fragmentation (autotomy) and regeneration. As sexual reproduction can also be induced, it is a good animal model for the study of both somatic and germline stem cells. To clarify the features of stem cells in regeneration, we investigated the proliferation and lineage of stem cells in E. japonensis. Neoblasts, which have the morphological characteristics of undifferentiated cells, were found to firmly adhere to the posterior surface of septa in each trunk segment. Also, smaller neoblast-like cells, which are designated as N-cells in this study, were located dorsal to the neoblasts on the septa. By conducting 5-bromo-2'-deoxyuridine (BrdU)-labeling-experiments, we have shown that neoblasts are slow-cycling (or quiescent) in intact growing worms, but proliferate rapidly in response to fragmentation. N-cells proliferate more actively than do neoblasts in intact worms. The results of pulse-chase experiments indicated that neoblast and N-cell lineage mesodermal cells that incorporated BrdU early in regeneration migrated toward the autotomized site to form the mesodermal region of the blastema, while the epidermal and intestinal cells also contributed to the blastema locally near the autotomized site. We have also shown that neoblasts have stem cell characteristics by expressing Ej-vlg2 and by the activity of telomerase during regeneration. Telomerase activity was high in the early stage of regeneration and correlated with the proliferation activity in the neoblast lineage of mesodermal stem cells. Taken together, our results indicate that neoblasts are mesodermal stem cells involved in the regeneration of E. japonensis.

Evolution and development of polarized germ cell cysts: new insights from a polychaete worm, Ophryotrocha labronica

Polarized oogenic cysts are clonal syncytia of germ cells in which some of the sister cells (cystocytes) differentiate not as oocytes, but instead as nurse cells: polyploid cells that support oocyte development. The intricate machinery required to establish and maintain divergent cell fates within a syncytium, and the importance of associated oocyte patterning for subsequent embryonic development, have made polarized cysts valuable subjects of study in developmental and cell biology. Nurse cell/oocyte specification is best understood in insects, particularly Drosophila melanogaster. However, polarized cysts have evolved independently in several other animal phyla. We describe the differentiation of female cystocytes in an annelid worm, the polychaete Ophryotrocha labronica. These worms are remarkable for their elegantly simple cysts, which comprise a single oocyte and nurse cell, making them an appealing complement to insects as subjects of study. To elucidate the process of cystocyte differentiation in O. labronica, we have constructed digital 3D models from electron micrographs of serially sectioned ovarian tissue. These models show that 2-cell cysts arise by fragmentation of larger "parental" cysts, rather than as independent units. The parental cysts vary in size and organization, are produced by asynchronous, indeterminate mitotic divisions of progenitor cystoblasts, and lack fusome-like organizing organelles. All of these characteristics represent key cytological differences from "typical" cyst development in insects like D. melanogaster. In light of such differences and the plasticity of female cyst structure among other animals, we suggest that it is time to reassess common views on the conservation of oogenic cysts and the importance of cysts in animal oogenesis generally.

Somatic and germline expression of piwi during development and regeneration in the marine polychaete annelid Capitella teleta

Background

Stem cells have a critical role during adult growth and regeneration. Germline stem cells are specialized stem cells that produce gametes during sexual reproduction. Capitella teleta (formerly Capitella sp. I) is a polychaete annelid that reproduces sexually, exhibits adult growth and regeneration, and thus, is a good model to study the relationship between somatic and germline stem cells.

Results

We characterize expression of the two C. teleta orthologs of piwi, genes with roles in germline development in diverse organisms. Ct-piwi1 and Ct-piwi2 are expressed throughout the life cycle in a dynamic pattern that includes both somatic and germline cells, and show nearly identical expression patterns at all stages examined. Both genes are broadly expressed during embryonic and larval development, gradually becoming restricted to putative primordial germ cells (PGCs) and the posterior growth zone. In juveniles, Ct-piwi1 is expressed in the presumptive gonads, and in reproductive adults, it is detected in gonads and the posterior growth zone. In addition, Ct-piwi1 is expressed in a population of putative PGCs that persist in sexually mature adults, likely in a stem cell niche. Ct-piwi1 is expressed in regenerating tissue, and once segments differentiate, it becomes most prominent in the posterior growth zone and immature oocytes in regenerating ovaries of regenerating segments.

Conclusions

In C. teleta, piwi genes may have retained an ancestral role as genetic regulators of both somatic and germline stem cells. It is likely that piwi genes, and associated stem cell co-regulators, became restricted to the germline in some taxa during the course of evolution.

Piwi-expressing hemoblasts serve as germline stem cells during postembryonic germ cell specification in colonial ascidian, Botryllus primigenus

Animals that propagate asexually are exciting models to investigate the cellular system, which produces germline cells constitutively throughout life. The present research investigated whether piwi was a germline-specific marker in the colonial ascidian Botryllus primigenus. An approximately 2.8 kb long cDNA fragment was cloned and termed BpPiwi, since the obtained amino acid sequence (874 aa) contained PAZ and PIWI domains. BpPiwi was expressed specifically by germline cells such as the loose cell mass (germline precursor cells), oocytes, spermatogonia, and spermatocytes. In addition, BpPiwi transcripts were also detected in some coelomic cells in the hemocoel and tunic vessels. BpPiwi(+) coelomic cells possessed similar morphological features to hemoblasts (stem cells). The concentration of BpPiwi(+) cells was found to be significantly lower than that obtained for hemoblasts suggesting that BpPiwi(+) cells comprise a fraction of hemoblasts. Further, the ability of BpPiwi(+) cells to serve as somatic stem cells was examined. No BpPiwi signals were detected from somatic hemoblasts forming vascular buds. The genetic knockdown of BpPiwi induced by siRNA injection resulted in the formation of a defective germline precursor. These results suggest that BpPiwi(+) hemoblasts reside in the hemocoel and tunic vessels and function as germline stem cells in the postembryonic colony. Based on the findings of the characterization of three effective germline genes piwi, vasa, and nanos, we propose that germline stem cells reside as BpPiwi(+)/BpVas(-)/BpNos(+) hemoblasts in B. primigenus.

Stem cell system in asexual and sexual reproduction of Enchytraeus japonensis (Oligochaeta, Annelida)

Enchytraeus japonensis is a small oligochaete species that proliferates asexually via fragmentation and regeneration. As sexual reproduction can also be induced, it is a good model system for the study of both regenerative and germline stem cells. It has been shown by histological study that putative mesodermal stem cells called neoblasts, and dedifferentiated epidermal and endodermal cells are involved in blastema formation. Recently, we isolated three region-specific marker genes expressed in the digestive tract and showed by in situ hybridization that morphallactic as well as epimorphic regulation of the body patterning occurs during regeneration. We also cloned two vasa-related genes and analyzed their expression during development and in mature worms that undergo sexual reproduction. The results arising form these studies suggest that the origin and development of germline stem cells and neoblasts may be independent. Furthermore, we carried out functional analysis using RNA interference (RNAi) and showed that a novel gene termed grimp is required for mesodermal cell proliferation at the initial stages of regeneration. These findings indicate that the stem cell system in E. japonensis is regulated by both internal and external environmental factors.

Stem cells are differentially regulated during development, regeneration and homeostasis in flatworms

The flatworm stem cell system is exceptional within the animal kingdom, as totipotent stem cells (neoblasts) are the only dividing cells within the organism. In contrast to most organisms, piwi-like gene expression in flatworms is extended from germ cells to somatic stem cells. We describe the isolation and characterization of the piwi homologue macpiwi in the flatworm Macrostomum lignano. We use in situ hybridization, antibody staining and RNA interference to study macpiwi expression and function in adults, during postembryonic development, regeneration and upon starvation. We found novelties regarding piwi function and observed differences to current piwi functions in flatworms. First, macpiwi was essential for the maintenance of somatic stem cells in adult animals. A knock-down of macpiwi led to a complete elimination of stem cells and death of the animals. Second, the regulation of stem cells was different in adults and regenerates compared to postembryonic development. Third, sexual reproduction of M. lignano allowed to follow germline formation during postembryonic development, regeneration, and starvation. Fourth, piwi expression in hatchlings further supports an embryonic formation of the germline in M. lignano. Our findings address new questions in flatworm stem cell research and provide a basis for comparison with higher organisms.

Exploration of embryonic origins of germline stem cells and neoblasts in Enchytraeus japonensis (Oligochaeta, Annelida)

An oligochaete annelid species, Enchytraeus japonensis, reproduces not only asexually but also sexually. It has been reported that putative mesodermal stem cells called neoblasts contribute to blastema formation and that Ej-piwi(+) germline stem cells participate in gonadal regeneration. To delineate the origin and formation of both of these stem cells, we isolated two vasa-related genes (Ej-vlg1 and Ej-vlg2) and analyzed the expression of each along with that of germline marker gene Ej-piwi. In adults, Ej-vlg1 and Ej-vlg2 were expressed in Ej-piwi(+) germline stem cells and germ cells in gonads, while only Ej-vlg2 mRNAs were detected in neoblasts. Expression analysis during embryogenesis indicated that clusters of Ej-vlg1(+)/Ej-vlg2(+) cells, located at the posterior ventral region in late embryos, became Ej-vlg1(+)/Ej-vlg2(+)/Ej-piwi(+) germline stem cells just after embryogenesis. On the other hand, Ej-vlg2 single positive cells with morphological characteristics of neoblasts became detectable much later after embryogenesis at the ventral position on each septum where adult neoblasts exist, although these early detected cells were much smaller in size than adult neoblasts. The present results suggest that (1) germline stem cells specified just after embryogenesis are derived from Ej-vlg1(+)/Ej-vlg2(+) cells which appear at the posterior ventral region in late embryos, and that (2) neoblasts appear much later in development.

Identification and origin of the germline stem cells as revealed by the expression of nanos-related gene in planarians

The planarian's remarkable regenerative ability is thought to be supported by the stem cells (neoblasts) found throughout its body. Here we report the identification of a subpopulation of neoblasts, which was revealed by the expression of the nanos-related gene of the planarian Dugesia japonica, termed Djnos. Djnos-expressing cells in the asexual planarian were distributed to the prospective ovary or testes forming region in the sexual planarian. During sexualization, Djnos-expressing cells produce germ cells, suggesting that in the asexual state these cells were kept as germline stem cells for the oogonia and spermatogonia. Interestingly, the germline stem cells were indistinguishable from the neoblasts by morphology and X-ray sensitivity and did not seem to contribute to the regeneration at all. Germline stem cells initially appear in the growing infant planarian, suggesting that germline stem cells are separated from somatic stem cells in the planarian. Thus, planarian neoblasts can be classified into two groups; somatic stem cells for regeneration and tissue renewal, and germline stem cells for production of germ cells during sexualization. However, Djnos-positive cells appeared in the newly formed trunk region from the head piece, suggesting that somatic stem cells can convert to germline stem cells.

Argonaute: A scaffold for the function of short regulatory RNAs

Argonaute is the central protein component of RNA-silencing mechanisms. It provides the platform for target-mRNA recognition by short regulatory guide RNA strands and the Slicer catalytic activity for mRNA cleavage in RNA interference. Multiple Argonaute sub-families can be identified phylogenetically yet, despite this diversity, molecular and sequence analyses show that Argonaute proteins share common molecular properties and the capacity to function through a common mechanism. Recently, the members of the Piwi sub-family have been shown to interact with new classes of short regulatory RNAs, Piwi-interacting RNAs (piRNAs) and repeat-associated small interfering RNAs (rasiRNAs), which has implications for developmental processes and introduces a new dimension to the field of RNA silencing.

Germline regeneration: the worms' turn

Asexual reproduction in the annelid Enchytraeus japonensis entails the regeneration of primordial germ cells from body parts that lack gonads. New primordial germ cells arise from piwi-expressing germline stem cells that are distinct from somatic stem cells.