Putative stem cells in the hemolymph and in the intestinal submucosa of the solitary ascidian Styela plicata

Comparing dormancy in two distantly related tunicates reveals morphological, molecular, and ecological convergences and repeated co-option

Many asexually-propagating marine invertebrates can survive extreme environmental conditions by developing dormant structures, i.e., morphologically simplified bodies that retain the capacity to completely regenerate a functional adult when conditions return to normal. Here, we examine the environmental, morphological, and molecular characteristics of dormancy in two distantly related clonal tunicate species: Polyandrocarpa zorritensis and Clavelina lepadiformis. In both species, we report that the dormant structures are able to withstand harsher temperature and salinity conditions compared to the adults. The dormant structures are the dominant forms these species employ to survive adverse conditions when the zooids themselves cannot survive. While previous work shows C. lepadiformis dormant stage is present in winters in the Atlantic Ocean and summers in the Mediterranean, this study is the first to show a year-round presence of P. zorritensis dormant forms in NW Italy, even in the late winter when all zooids have disappeared. By finely controlling the entry and exit of dormancy in laboratory-reared individuals, we were able to select and characterize the morphology of dormant structures associated with their transcriptome dynamics. In both species, we identified putative stem and nutritive cells in structures that resemble the earliest stages of asexual propagation. By characterizing gene expression during dormancy and regeneration into the adult body plan (i.e., germination), we observed that genes which control dormancy and environmental sensing in other metazoans, notably HIF-α and insulin signaling genes, are also expressed in tunicate dormancy. Germination-related genes in these two species, such as the retinoic acid pathway, are also found in other unrelated clonal tunicates during asexual development. These results are suggestive of repeated co-option of conserved eco-physiological and regeneration programs for the origin of novel dormancy-germination processes across distantly related animal taxa.

Central nervous system regeneration in ascidians: cell migration and differentiation

Adult ascidians have the capacity to regenerate the central nervous system (CNS) and are therefore excellent models for studies on neuroregeneration. The possibility that undifferentiated blood cells are involved in adult neuroregeneration merits investigation. We analyzed the migration, circulation, and role of hemocytes of the ascidian Styela plicata in neuroregeneration. Hemocytes were removed and incubated with superparamagnetic iron oxide nanoparticles (SPION), and these SPION-labeled hemocytes were injected back into the animals (autologous transplant), followed by neurodegeneration with the neurotoxin 3-acetylpyridine (3AP). Magnetic resonance imaging showed that 1, 5, and 10 days after injury, hemocytes migrated to the intestinal region, siphons, and CNS. Immunohistochemistry revealed that the hemocytes that migrated to the CNS were putative stem cells (P-element-induced wimpy testis + or PIWI + cells). In the cortex of the neural ganglion, migrated hemocytes started to lose their PIWI labeling 5 days after injury, and 10 days later started to show β-III tubulin labeling. In the neural gland, however, the hemocytes remained undifferentiated during the entire experimental period. Transmission electron microscopy revealed regions in the neural gland with characteristics of neurogenic niches, not previously reported in ascidians. These results showed that migration of hemocytes to the hematopoietic tissue and to the 3AP-neurodegenerated region is central to the complex mechanism of neuroregeneration.

Searching for the Origin and the Differentiation of Haemocytes before and after Larval Settlement of the Colonial Ascidian Botryllus schlosseri: An Ultrastructural Viewpoint

The colonial ascidian Botryllus schlosseri possesses an innate immunity, which plays fundamental roles in its survival, adaptability, worldwide spread and ecological success. Three lines of differentiation pathways of circulating haemocytes are known to be present in the haemolymph, starting from undifferentiated haemoblasts: (i) the phagocytic line (hyaline amoebocytes and macrophage-like cells), (ii) the cytotoxic line (granular amoebocytes and morula cells) and (iii) the storage cell line (pigment cells and nephrocytes). Many questions remain about their origin, and thus, observations during various stages of development were undertaken in this study. Haemocytes were detected beginning from the early tailbud embryo stage. Haemoblasts were always present and morula cells were the first differentiated haemocytes detected. In both the next stage, just before hatching, and the swimming tadpole larva stage, hyaline amoebocytes and pigment cells were also recognisable. Some morula cells containing active phenoloxidase migrated from the haemolymph into the tunic after having crossed the epidermis, and this behaviour could be related to the preparation of a defensive function for spatial competition. During larval metamorphosis, macrophage-like cells appeared with their phagosomes positive to acid phosphatase activity and containing apoptotic cells from tail tissue degeneration. After metamorphosis, in the filter-feeding oozoid stage, nephrocytes involved in nitrogen catabolism finally appeared. In both the subendostylar sinus and the peripheral blind-sac vessels (ampullae), clusters of haemoblasts were recognisable, some of which showed incipient specialisations, considering the hypothesis of the presence of putative niches of haemolymph stem cells.

Localization of vasoactive intestinal peptide and toll-like receptor 2 immunoreactive cells in endostyle of urochordate Styela plicata (Lesueur, 1823)

The endostyle is the first component of the ascidian digestive tract, it is shaped like a through and is located in the pharynx's ventral wall. This organ is divided longitudinally into nine zones that are parallel to each other. Each zone's cells are physically and functionally distinct. Support elements are found in zones 1, 3, and 5, while mucoproteins secreting elements related to the filtering function are found in zones 2, 4, and 6. Zones 7, 8, and 9, which are located in the lateral dorsal section of the endostyle, include cells with high iodine and peroxidase concentrations. Immunohistochemical technique using the following antibodies, Toll‐like receptor 2 (TLR‐2) and vasoactive intestinal peptide (VIP), and lectin histochemistry (WGA—wheat‐germagglutinin), were used in this investigation to define immune cells in the endostyle of Styela plicata (Lesueur, 1823). Our results demonstrate the presence of immune cells in the endostyle of S. plicata, highlighting that innate immune mechanisms are highly conserved in the phylogeny of the chordates.

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.

Articulating the "stem cell niche" paradigm through the lens of non-model aquatic invertebrates

Stem cells (SCs) in vertebrates typically reside in "stem cell niches" (SCNs), morphologically restricted tissue microenvironments that are important for SC survival and proliferation. SCNs are broadly defined by properties including physical location, but in contrast to vertebrates and other "model" organisms, aquatic invertebrate SCs do not have clearly documented niche outlines or properties. Life strategies such as regeneration or asexual reproduction may have conditioned the niche architectural variability in aquatic or marine animal groups. By both establishing the invertebrates SCNs as independent types, yet allowing inclusiveness among them, the comparative analysis will allow the future functional characterization of SCNs.

Toll-like receptor 2 and α-Smooth Muscle Actin expressed in the tunica of a urochordate, Styela plicata

The tunicate, Styela plicata (Lesueur, 1823) present an open circulator system with a tubular heart and blood flowing in lacunae among organs, bathing the tissues directly. Blood vascular lacunae are present in the tunica that is situated outside the epidermis and present a fibrous structure. The cells of the tunic are in straight contact with the blood vessels or are highly mobile. Ascidians are considered model organisms in comparative immunology of the chordate, and hold an important phylogenetic position as sister group of vertebrates. In recent years, numerous studies have reported the presence of Toll-like receptors (TLRs) in the genome of non-mammalian organisms including invertebrates. Two TLRs, designated Ci-TLR1 and Ci-TLR2 were expressed in the stomach, intestine and in numerous hemocytes of Ciona intestinalis, demonstrating that these key transmembrane proteins are evolutionarily conserved in ascidians. In this study for the first time, hemocytes aggregates were identified by confocal immunofluorescence techniques, using TLR2 antibody in the tunica of Styela plicata; furthermore, α-Smooth Muscle Actin (α-SMA) expression has been shown in the cells lining the vessels of the tunic. Our results support the view that the TLR-mediated innate immune functions are conserved in ascidian tissues.

Crayfish hemocytes develop along the granular cell lineage

Despite the central role of hemocytes in crustacean immunity, the process of hemocyte differentiation and maturation remains unclear. In some decapods, it has been proposed that the two main types of hemocytes, granular cells (GCs) and semigranular cells (SGCs), differentiate along separate lineages. However, our current findings challenge this model. By tracking newly produced hemocytes and transplanted cells, we demonstrate that almost all the circulating hemocytes of crayfish belong to the GC lineage. SGCs and GCs may represent hemocytes of different developmental stages rather than two types of fully differentiated cells. Hemocyte precursors produced by progenitor cells differentiate in the hematopoietic tissue (HPT) for 3 ~ 4 days. Immature hemocytes are released from HPT in the form of SGCs and take 1 ~ 3 months to mature in the circulation. GCs represent the terminal stage of development. They can survive for as long as 2 months. The changes in the expression pattern of marker genes during GC differentiation support our conclusions. Further analysis of hemocyte phagocytosis indicates the existence of functionally different subpopulations. These findings may reshape our understanding of crustacean hematopoiesis and may lead to reconsideration of the roles and relationship of circulating hemocytes.

Stem cells of aquatic invertebrates as an advanced tool for assessing ecotoxicological impacts

Environmental stressors are assessed through methods that quantify their impacts on a wide range of metrics including species density, growth rates, reproduction, behaviour and physiology, as on host-pathogen interactions and immunocompetence. Environmental stress may induce additional sublethal effects, like mutations and epigenetic signatures affecting offspring via germline mediated transgenerational inheritance, shaping phenotypic plasticity, increasing disease susceptibility, tissue pathologies, changes in social behaviour and biological invasions. The growing diversity of pollutants released into aquatic environments requires the development of a reliable, standardised and 3R (replacement, reduction and refinement of animals in research) compliant in vitro toolbox. The tools have to be in line with REACH regulation 1907/2006/EC, aiming to improve strategies for potential ecotoxicological risks assessment and monitoring of chemicals threatening human health and aquatic environments. Aquatic invertebrates' adult stem cells (ASCs) are numerous and can be pluripotent, as illustrated by high regeneration ability documented in many of these taxa. This is of further importance as in many aquatic invertebrate taxa, ASCs are able to differentiate into germ cells. Here we propose that ASCs from key aquatic invertebrates may be harnessed for applicable and standardised new tests in ecotoxicology. As part of this approach, a battery of modern techniques and endpoints are proposed to be tested for their ability to correctly identify environmental stresses posed by emerging contaminants in aquatic environments. Consequently, we briefly describe the current status of the available toxicity testing and biota-based monitoring strategies in aquatic environmental ecotoxicology and highlight some of the associated open issues such as replicability, consistency and reliability in the outcomes, for understanding and assessing the impacts of various chemicals on organisms and on the entire aquatic environment. Following this, we describe the benefits of aquatic invertebrate ASC-based tools for better addressing ecotoxicological questions, along with the current obstacles and possible overhaul approaches.

Comparative study of coelomocytes from Arbacia lixula and Lythechinus variegatus: Cell characterization and in vivo evidence of the physiological function of vibratile cells

The knowledge on echinoderm coelomocytes has increased in recent years, but researchers still face a complex problem: how to obtain purified cells. Even flow cytometry being useful to address coelomocytes in suspension, the need for a method able to provide isolated cells is still noteworthy. Here, we use Imaging Flow Cytometry (IFC) to characterize the coelomocytes of two sea urchin species - Arbacia lixula and Lytechinus variegatus - and obtain gates to isolate cell populations. Then, we used these gates to study the physiological response of A. lixula coelomocytes during an induced immune challenge with Escherichia coli. An analysis of area and aspect ratio parameters of the flow cytometer allowed the identification of two main cell populations in the coelomic fluid: circular and elongated cells. A combination of this method with nucleus labeling using propidium iodide allowed the determination of gates containing isolated subpopulations of vibratile cells, red spherulocytes, and two phagocytes subpopulations in both species. We observed that during an induced bacterial immune challenge, A. lixula was able to modulate coelomocyte frequencies, increasing the phagocytes and decreasing red spherulocytes and vibratile cells. These results indicate that vibratile cells and red spherulocytes act by immobilizing and stoping bacterial growth, respectively, cooperating with phagocytes in the immune response. The use of IFC was fundamental not only to identify specific gates for the main coelomic subpopulations but also allowed the investigation on how echinoids modulate their physiological responses during immune challenges. Furthermore, we provide the first experimental evidence about the role of vibratile cells, corroborating its involvement with the immune system.

Anti-virus effects of interferon regulatory factors (IRFs) identified in ascidian Ciona savignyi

Interferon regulatory factors (IRFs) are key transcription factors that function in the immune system via the interferon (IFN) pathway. In the current study, we identified and characterized three IRFs (CsIRFL1, CsIRFL2, and CsIRFL3) from ascidian Ciona savignyi. Phylogenetic analysis showed that CsIRFL1 was clustered with two IRFs from Ciona robusta and shrimp IRF apart from the vertebrate IRFs, whereas CsIRFL2 and CsIRFL3 were grouped with an unnamed protein from Oikopleura dioica into a sub-branch highly identifying with the vertebrate IRF4, IRF8, and IRF9. Gene expression analysis revealed that CsIRFL1 and CsIRFL2 expressed in all the examined adult tissues (stomach, intestines, eggs, hemocytes, gonad, heart, and pharynx) and predominantly in hemocytes. However, the expression of CsIRFL3 was undetectable in the tested adult tissues. Furthermore, in situ hybridization showed that CsIRFL1 and CsIRFL2 mainly expressed in immunocytes within hemolymph, including phagocytes, macrophage-like cells, morula cells, and amoebocytes, suggesting CsIRFL1 and CsIRFL2 were involved in ascidian immune responses. We then performed LPS and poly(I:C) challenge assay and found that CsIRFL1 highly expressed in the cultured hemocytes following LPS infection for 24 h. After viral analogue poly(I:C) stimulation, the expression of CsIRFL2 was dramatically upregulated from 12 to 24 h. Meanwhile, two critical components of the IFN signaling pathways, STAT and TBK1, showed the increased expression as well after poly(I:C) induction, indicating that CsIRFL2 and IFN pathways genes were activated under the infection of viral analogue. Thus, our findings suggested that CsIRFL2 was a potential transcriptional regulatory factor that participated in regulating the ascidian anti-virus immune response.

Spawning induction, development and culturing of the solitary ascidian Polycarpa mytiligera, an emerging model for regeneration studies

BACKGROUND

Ascidians (phylum Chordata, class Ascidiacea) represent the closest living invertebrate relatives of the vertebrates and constitute an important model for studying the evolution of chordate development. The solitary ascidian Polycarpa mytiligera exhibits a robust regeneration ability, unique among solitary chordates, thus offering a promising new model for regeneration studies. Understanding its reproductive development and establishing land-based culturing methods is pivotal for utilizing this species for experimental studies. Its reproduction cycle, spawning behavior, and developmental processes were therefore studied in both the field and the lab, and methods were developed for its culture in both open and closed water systems.

RESULTS

Field surveys revealed that P. mytiligera's natural recruitment period starts in summer (June) and ends in winter (December) when seawater temperature decreases. Laboratory experiments revealed that low temperature (21 °C) has a negative effect on its fertilization and development. Although spontaneous spawning events occur only between June and December, we were able to induce spawning under controlled conditions year-round by means of gradual changes in the environmental conditions. Spawning events, followed by larval development and metamorphosis, took place in ascidians maintained in either artificial or natural seawater facilities. P. mytiligera's fast developmental process indicated its resemblance to other oviparous species, with the larvae initiating settlement and metamorphosis at about 12 h post-hatching, and reaching the juvenile stage 3 days later.

CONCLUSIONS

Polycarpa mytiligera can be induced to spawn in captivity year-round, independent of the natural reproduction season. The significant advantages of P. mytiligera as a model system for regenerative studies, combined with the detailed developmental data and culturing methods presented here, will contribute to future research addressing developmental and evolutionary questions, and promote the use of this species as an applicable model system for experimental studies.