Muscle cell differentiation in the ascidian heart

Pulsation waves along the Ciona heart tube reverse by bimodal rhythms expressed by a remote pair of pacemakers

The heart of ascidians (marine invertebrate chordates) has a tubular structure, and heartbeats propagate from one end to the other. The direction of pulsation waves intermittently reverses in the heart of ascidians and their relatives; however, the underlying mechanisms remain unclear. We herein performed a series of experiments to characterize the pacemaker systems in isolated hearts and their fragments, and applied a mathematical model to examine the conditions leading to heart reversals. The isolated heart of Ciona robusta autonomously generated pulsation waves at ∼20 to 25 beats min-1 with reversals at ∼1 to 10 min intervals. Experimental bisections of isolated hearts revealed that independent pacemakers resided on each side and also that their beating frequencies periodically changed as they expressed bimodal rhythms, which comprised an ∼1.25 to 5.5 min acceleration/deceleration cycle of a beating rate of between 0 and 25 beats min-1. Only fragments including 5% or shorter terminal regions of the heart tube maintained autonomous pulsation rhythms, whereas other regions did not. Our mathematical model, based on FitzHugh-Nagumo equations applied to a one-dimensional alignment of cells, demonstrated that the difference between frequencies expressed by the two independent terminal pacemakers determined the direction of propagated waves. Changes in the statuses of terminal pacemakers between the excitatory and oscillatory modes as well as in their endogenous oscillation frequencies were sufficient to lead to heart reversals. These results suggest that the directions of pulsation waves in the Ciona heart reverse according to the changing rhythms independently expressed by remotely coupled terminal pacemakers.

Regulation and evolution of muscle development in tunicates

For more than a century, studies on tunicate muscle formation have revealed many principles of cell fate specification, gene regulation, morphogenesis, and evolution. Here, we review the key studies that have probed the development of all the various muscle cell types in a wide variety of tunicate species. We seize this occasion to explore the implications and questions raised by these findings in the broader context of muscle evolution in chordates.

Novel budding mode in Polyandrocarpa zorritensis: a model for comparative studies on asexual development and whole body regeneration

Background

In tunicates, the capacity to build an adult body via non-embryonic development (NED), i.e., asexual budding and whole body regeneration, has been gained or lost several times across the whole subphylum. A recent phylogeny of the family Styelidae revealed an independent acquisition of NED in the colonial species Polyandrocarpa zorritensis and highlighted a novel budding mode. In this paper, we provide the first detailed characterization of the asexual life cycle of P. zorritensis.

Results

Bud formation occurs along a tubular protrusion of the adult epidermis, the stolon, in a vascularized area defined as budding nest. The bud arises through a folding of the epithelia of the stolon with the contribution of undifferentiated mesenchymal cells. This previously unreported mode of bud onset leads to the formation of a double vesicle, which starts to develop into a zooid through morphogenetic mechanisms common to other Styelidae. The budding nest can also continue to accumulate nutrients and develop into a round-shaped structure, designated as spherule, which represents a dormant form able to survive low temperatures.

Conclusions

To understand the mechanisms of NED and their evolution, it is fundamental to start from a robust phylogenetic framework in order to select relevant species to compare. The anatomical description of P. zorritensis NED provides the foundation for future comparative studies on plasticity of budding and regeneration in tunicates.

Electronic supplementary material

The online version of this article (10.1186/s13227-019-0121-x) contains supplementary material, which is available to authorized users.

Modular co-option of cardiopharyngeal genes during non-embryonic myogenesis

Background

In chordates, cardiac and body muscles arise from different embryonic origins. In addition, myogenesis can be triggered in adult organisms, during asexual development or regeneration. In non-vertebrate chordates like ascidians, muscles originate from embryonic precursors regulated by a conserved set of genes that orchestrate cell behavior and dynamics during development. In colonial ascidians, besides embryogenesis and metamorphosis, an adult can propagate asexually via blastogenesis, skipping embryo and larval stages, and form anew the adult body, including the complete body musculature.

Results

To investigate the cellular origin and mechanisms that trigger non-embryonic myogenesis, we followed the expression of ascidian myogenic genes during Botryllus schlosseri blastogenesis and reconstructed the dynamics of muscle precursors. Based on the expression dynamics of Tbx1/10, Ebf, Mrf, Myh3 for body wall and of FoxF, Tbx1/10, Nk4, Myh2 for heart development, we show that the embryonic factors regulating myogenesis are only partially co-opted in blastogenesis, and that markers for muscle precursors are expressed in two separate domains: the dorsal tube and the ventral mesenchyma.

Conclusions

Regardless of the developmental pathway, non-embryonic myogenesis shares a similar molecular and anatomical setup as embryonic myogenesis, but implements a co-option and loss of molecular modules. We then propose that the cellular precursors contributing to heart and body muscles may have different origins and may be coordinated by different developmental pathways.

Electronic supplementary material

The online version of this article (10.1186/s13227-019-0116-7) contains supplementary material, which is available to authorized users.

The role of the pericardium in the valveless, tubular heart of the tunicate Ciona savignyi

Tunicates, small invertebrates within the phylum Chordata, possess a robust tubular heart which pumps blood through their open circulatory systems without the use of valves. This heart consists of two major components: the tubular myocardium, a flexible layer of myocardial cells that actively contracts to drive fluid down the length of the tube; and the pericardium, a stiff, outer layer of cells that surrounds the myocardium and creates a fluid-filled space between the myocardium and the pericardium. We investigated the role of the pericardium through in vivo manipulations on tunicate hearts and computational simulations of the myocardium and pericardium using the immersed boundary method. Experimental manipulations reveal that damage to the pericardium results in aneurysm-like bulging of the myocardium and major reductions in the net blood flow and percentage closure of the heart's lumen during contraction. In addition, varying the pericardium-to-myocardium (PM) diameter ratio by increasing damage severity was positively correlated with peak dye flow in the heart. Computational simulations mirror the results of varying the PM ratio experimentally. Reducing the stiffness of the myocardium in the simulations reduced mean blood flow only for simulations without a pericardium. These results indicate that the pericardium has the ability to functionally increase the stiffness of the myocardium and limit myocardial aneurysms. The pericardium's function is likely to enhance flow through the highly resistive circulatory system by acting as a support structure in the absence of connective tissue within the myocardium.

Histone methylation codes involved in stemness, multipotency, and senescence in budding tunicates

We examined the dynamics of nuclear histone H3 trimethylation related to cell differentiation and aging in a budding tunicate, Polyandrocarpa misakiensis. Throughout zooidal life, multipotent epithelial and coelomic cell nuclei showed strong trimethylation signals at H3 lysine27 (H3K27me3), consistent with the results of western blotting. Epidermal H3K27me3 repeatedly appeared in protruding buds and disappeared in senescent adult zooids. The budding-specific cytostatic factor TC14-3 allowed aging epidermal cells to restore H3K27me3 signals and mitochondrial gene activities via mitochondrial transcription factor a, all of which were made ineffective by an H3K27me3 inhibitor. Chromatin immunoprecipitation showed that TC14-3 enhances H3K27me3 of transdifferentiation-related genes and consequently downregulates the expression of these genes. In contrast, trimethylation signals at H3 lysine4 (H3K4me3) appeared transiently in transdifferentiating bud cells and stably lasted in undifferentiated adult cells without affecting H3K27me3. A transdifferentiation-related gene external signal-regulated kinase heavily underwent H3K4me3 in developing buds, which could be reproduced by retinoic acid. These results indicate that in P. misakiensis, TC14-3-driven H3K27 trimethylation is a default state of bud and zooid cells, which serves as the histone code for cell longevity. H3K27me3 and H3K4me3 double-positive signals are involved in cell stemness, and absence of signals is the indication of senescence.

Ontology for the asexual development and anatomy of the colonial chordate Botryllus schlosseri

Ontologies provide an important resource to integrate information. For developmental biology and comparative anatomy studies, ontologies of a species are used to formalize and annotate data that are related to anatomical structures, their lineage and timing of development. Here, we have constructed the first ontology for anatomy and asexual development (blastogenesis) of a bilaterian, the colonial tunicate Botryllus schlosseri. Tunicates, like Botryllus schlosseri, are non-vertebrates and the only chordate taxon species that reproduce both sexually and asexually. Their tadpole larval stage possesses structures characteristic of all chordates, i.e. a notochord, a dorsal neural tube, and gill slits. Larvae settle and metamorphose into individuals that are either solitary or colonial. The latter reproduce both sexually and asexually and these two reproductive modes lead to essentially the same adult body plan. The Botryllus schlosseri Ontology of Development and Anatomy (BODA) will facilitate the comparison between both types of development. BODA uses the rules defined by the Open Biomedical Ontologies Foundry. It is based on studies that investigate the anatomy, blastogenesis and regeneration of this organism. BODA features allow the users to easily search and identify anatomical structures in the colony, to define the developmental stage, and to follow the morphogenetic events of a tissue and/or organ of interest throughout asexual development. We invite the scientific community to use this resource as a reference for the anatomy and developmental ontology of B. schlosseri and encourage recommendations for updates and improvements.

Characterization of HCN and cardiac function in a colonial ascidian

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels generate the rhythmic beating of mammalian hearts. We identified an HCN homolog in the colonial ascidian Botryllus schlosseri, a nonvertebrate chordate which possesses a tubular heart that beats bidirectionally. Contractions initiate at one end of the heart and travel across the length of the organ, and these periodically reverse, suggesting the presence of two pacemakers, one on each side. We find that HCN expression is highly enriched in cells scattered throughout the myocardium. We functionally analyzed the role of HCN channels in heartbeat using the antagonists Cilobradine and Zatebradine, which decreased the heartbeat in a reversible manner. We also assessed the role of β-adrenoreceptors in regulating HCN function using the antagonist Metoprolol, which lowered heartbeat rate (HR), as well as the agonist Isoproterenol, which did not alter HR, but caused simultaneous beating, analogous to a fibrillation. Measurements of direction and velocity of blood flow by making use of a novel system to study heart function in model systems amenable to live imaging revealed a significant correlation between heartbeat arrhythmia and drug treatment, similar to that observed with the same drugs in vertebrates. These results suggest that the heart pacemaker in tunicates may be homologous to that in their vertebrate counterparts in both development and function.

Relationships among hemocytes, tunic cells, germ cells, and accessory cells in the colonial ascidian Botryllus schlosseri

Monoclonal antibodies were raised against hemocytes of the colonial ascidian Botryllus schlosseri as possible tools to study hemocyte differentiation. In this species, blood cells are involved in various biological functions, such as immunosurveillance, encapsulation of foreign bodies, metal accumulation, and allorecognition. The latter process drives the fusion or rejection of contacting colonies, according to whether they do or do not share at least one allele at the fusibility/histocompatibility (Fu/HC) locus. Hemocytes take part in the rejection reaction, which suggests that they express molecules, coded by the Fu/HC locus, on their surface. A homozygous colony at the Fu/HC locus was used to produce the antibodies, which were screened by immunocytochemistry on hemocyte monolayers, immunohistochemistry on colony paraffin sections, and immunoblotting on colony homogenates. Here, we report on one of the obtained antibodies (1D8), which recognized a surface epitope on hemocytes of the donor colony and other colonies, apparently in a manner specific to the Fu/HC genotype. It also labeled a single 80-kDa band in colony homogenates. In addition, it specifically recognized tunic cells, germ cells, and their accessory cells. These results strengthen the assumption of a close relationship among these types of cells and blood cells, and suggest a close relationship among the above cells, probably deriving from undifferentiated blood cells.

Muscle differentiation in a colonial ascidian: organisation, gene expression and evolutionary considerations

BACKGROUND

Ascidians are tunicates, the taxon recently proposed as sister group to the vertebrates. They possess a chordate-like swimming larva, which metamorphoses into a sessile adult. Several ascidian species form colonies of clonal individuals by asexual reproduction. During their life cycle, ascidians present three muscle types: striated in larval tail, striated in the heart, and unstriated in the adult body-wall.

RESULTS

In the colonial ascidian Botryllus schlosseri, we investigated organisation, differentiation and gene expression of muscle beginning from early buds to adults and during zooid regression. We characterised transcripts for troponin T (BsTnT-c), adult muscle-type (BsMA2) and cytoplasmic-type (BsCA1) actins, followed by in situ hybridisation (ISH) on sections to establish the spatio-temporal expression of BsTnT-c and BsMA2 during asexual reproduction and in the larva. Moreover, we characterised actin genomic sequences, which by comparison with other metazoans revealed conserved intron patterns.

CONCLUSION

Integration of data from ISH, phalloidin staining and TEM allowed us to follow the phases of differentiation of the three muscle kinds, which differ in expression pattern of the two transcripts. Moreover, phylogenetic analyses provided evidence for the close relationship between tunicate and vertebrate muscle genes. The characteristics and plasticity of muscles in tunicates are discussed.

Identification of the endostyle as a stem cell niche in a colonial chordate

Stem cell populations exist in "niches" that hold them and regulate their fate decisions. Identification and characterization of these niches is essential for understanding stem cell maintenance and tissue regeneration. Here we report on the identification of a novel stem cell niche in Botryllus schlosseri, a colonial urochordate with high stem cell-mediated developmental activities. Using in vivo cell labeling, engraftment, confocal microscopy, and time-lapse imaging, we have identified cells with stemness capabilities in the anterior ventral region of the Botryllus' endostyle. These cells proliferate and migrate to regenerating organs in developing buds and buds of chimeric partners but do not contribute to the germ line. When cells are transplanted from the endostyle region, they contribute to tissue development and induce long-term chimerism in allogeneic tissues. In contrast, cells from other Botryllus' regions do not show comparable stemness capabilities. Cumulatively, these results define the Botryllus' endostyle region as an adult somatic stem cell niche.

Cell proliferation dynamics of somatic and germline tissues during zooidal life span in the colonial tunicate Botryllus primigenus

Botryllus primigenus is a colonial tunicate in which three successive generations develop synchronously. To identify proliferation centers and possible adult stem cells during asexual reproduction, somatic and germline cells were labeled with 5-bromo-2'-deoxyuridine (BrdU). In the youngest generation, multipotent epithelial cells exhibited an average labeling index (LI) of 30% 24 hr after BrdU injection. In the middle generation, the LI of organ rudiments decreased gradually and reached zero by the beginning of the eldest generation. Exceptionally, cells of specialized tissues such as the pharyngeal inner longitudinal vessel and the posterior end of the endostyle continued DNA synthesis and mitosis even in the eldest generation. Proliferating somatic and germline cells of younger generations expressed a Botryllus myc homolog (BpMyc), but adult tissues did not. This result strongly suggests that in B. primigenus undifferentiated progenitor cells are discernible from possible adult stem cells by the presence or absence of BpMyc.

Body Muscle-Cell Differentiation from Coelomic Stem Cells in Colonial Tunicates

Body muscle-cell differentiation was ultrastructurally examined in palleal buds of the colonial tunicate Symplegma reptans. Undifferentiated coelomic cells accumulate near the primordial oral siphon and associate with the basal lamina beneath the epidermis. They initially display the characteristics of hemoblast cells that have a large nucleus with a prominent nucleolus and narrow cytoplasm filled with polysomes. However, they soon become unique due to the development of an indented contour of the nucleus. When the basal lamina of the epidermis develops into the fibrous extracellular matrix (ECM), the muscle precursor cell has the deeply-notched nucleus, and thick and thin filaments in the cytoplasm facing the ECM. Collagen fibril-like structures appear in the ECM. Myofilaments are arranged with the ratio of thick to thin filaments being 1:2.5. Dense bodies and plaques become evident before the oral siphon is perforated. These results show that in S. reptans, the sphincter muscle cells arise from undifferentiated hemoblasts, and that their differentiation begins with a morphological change in their nuclei. Epidermal cells and/or the ECM may have an inductive effect on muscle cell differentiation.

Botryllus schlosseri: a model ascidian for the study of asexual reproduction

Botryllus schlosseri, a cosmopolitan colonial ascidian reared in the laboratory for more than 50 years, reproduces both sexually and asexually and is used as a model organism for studying a variety of biological problems. Colonies are formed of numerous, genetically identical individuals (zooids) and undergo cyclical generation changes in which the adult zooids die and are replaced by their maturing buds. Because the progression of the colonial life cycle is intimately correlated with blastogenesis, a shared staging method of bud development is required to compare data coming from different laboratories. With the present review, we aim (1) to introduce B. schlosseri as a valuable chordate model to study various biological problems and, especially, sexual and asexual development; (2) to offer a detailed description of bud development up to adulthood and the attainment of sexual maturity; (3) to re-examine Sabbadin's (1955) staging method and re-propose it as a simple tool for in vivo recognition of the main morphogenetic events and recurrent changes in the blastogenetic cycle, as it refers to the developmental stages of buds and adults.

Common and divergent pathways in alternative developmental processes of ascidians

Colonial ascidians offer opportunities to investigate how developmental events are integrated to generate the animal form, since they can develop similar individuals (oozooids from eggs, blastozooids from pluripotent somatic cells) through very different reproductive processes, i.e. embryogenesis and blastogenesis. Moreover, thanks to their key phylogenetic position, they can help in the understanding of the molecular mechanisms of morphogenesis and their evolution in chordates. We review organogenesis of the ascidian neural complex comparing embryos and buds in terms of topology, developmental mechanisms and terminology. We propose a new interpretation of bud territories, and reconsider nervous system development based on recent results suggesting that ascidians have vertebrate placodal and neural-crest-like cells. Comparing embryonic and blastogenic development in Botryllus schlosseri, we propose that the bud has territories with a placodal potentiality, suggesting that chordate ancestors possessed neurogenic placodes, and that the genetic pathways regulating neurogenic placode formation were co-opted for new developmental processes, such as blastogenesis.

Proliferation and Differentiation Processes in the Heart Muscle Elements in Different Phylogenetic Groups

This article reviews, discusses, and summarizes data about the generative behavior of muscle tissue cells, the mechanisms of its regulation, and the organization of the endocrine function of the heart in the main phylogenetic groups. With respect to the ratio of processes of proliferation and differentiation, cell organization, and growth mechanism, muscle tissues of propulsive organs can be divided into three types, each revealed in one of three main groups of animals, lophotrochozoans, ecdysozoans, and chordates. Ecdysterone is likely to play the key role in the regulation of proliferation and differentiation processes in the heart muscle of crustaceans, and, most probably, also of molluscs. In each of the three main phylogenetic groups the endocrine function of the heart consisting of secretion of natriuretic peptides has a peculiar organization. Vertebrate cardiomyocytes are known to combine contractile and endocrine differentiation. Such functional dualism is absent in heart muscle elements of Lophotrochozoa and Ecdysozoa; in the heart of lopfotrochozoans, secretion of natriuretic peptides is performed by endothelial cells and their derivatives. Homology of the heart muscle in the animal kingdom as well as possible mechanisms of genomic and epigenomic regulation of different types of cardiomyogenesis are discussed.

Development of the motor nervous system in ascidians

The motor nervous system of adult ascidians consists of neurons forming the cerebral ganglion from which axons run out directly to the effectors, i.e., muscular and ciliary cells. In this study, we analyzed the development of the motor fibers, correlating this with organ differentiation during asexual reproduction in Botryllus schlosseri. We used a staining method for acetylcholinesterase, whose reaction product is visible with both light and electron microscopy and which labels entire nerves, including their thin terminals, making them identifiable between tissues. While the cerebral ganglion is forming, the axons elongate and follow stereotypical pathways to reach the smooth muscle cells of the body, the striated muscle of the heart, and the ciliated cells of the branchial stigmata and the gut. A strict temporal relation links the development of the local neural network with its target organ, which is approached by nerves before the effector cells are fully differentiated. This process occurs for oral and cloacal siphons, branchial basket, gut, and heart. Axons grow through the extracellular matrix and arrive at their targets from different directions. In some cases, the blood sinuses constitute the favorite roads for growing axons, which seem to be guided by a mechanism involving contact guidance or stereotropism. The pattern of innervation undergoes dynamic rearrangements and a marked process of elimination of axons, when the last stages of blastogenesis occur. The final pattern of motor innervation seems to be regulated by axon withdrawal, rather than apoptosis of motor neurons.

Cell adhesion in the process of asexual reproduction of tunicates

Cell adhesion during budding of tunicates is reviewed from the viewpoints of histology, cytology, biochemistry, and molecular biology. Two kinds of multipotent cells play important roles in bud formation and development: epithelial cells, such as the atrial epithelium of botryllids and polystyelids, and mesenchymal cells, referred to as haemoblasts. Haemoblasts are able to aggregate to form a solid mass of cells, which soon becomes a hollow vesicle. The vesicular epithelium has junctional complexes that contain adherens junctions, and, sometimes, tight junctions; both occur apicolaterally on the plasma membrane. The hollow vesicle develops into the heart, the pyloric gland and duct, the gonad, including germ cells, and even the multipotent epithelium of buds. Cell culture studies suggest that multipotent epithelial cells may be interchangeable with haemoblasts. Several kinds of calcium-dependent, galactose-binding tunicate lectins (TC-14s) have been isolated and sequenced, and have been found to facilitate both in vivo and in vitro cell aggregation and migration. Tunicate homologs of cadherin and integrin genes have recently been isolated from Botryllus and Polyandrocarpa, respectively. Their unique molecular characteristics are discussed in the context of roles that they play in cell adhesion in the process of tunicate budding.

Morphogenetic tissue interactions during posterior commitment in palleal buds of the polystyelid ascidian, Polyandrocarpa misakiensis

The development of chimeric double-anterior and double-posterior buds in the polystyelid ascidian, Polyandrocarpa misakiensis, was studied. Bud pieces used were allowed to develop for various time periods before the fusion operation. One-day double-half buds developed into complete zooids with normal anteroposterior polarity. In these cases, the bud half whose site of origin on the parent was higher (more posterior) than the partner formed the posterior organ, the digestive tract, of a resultant zooid. Two-day double-posterior buds developed into biposterior zooids that had two digestive tracts, whereas 2-day double-anterior buds still underwent pattern regulation in the same manner as 1-day buds (23-24 degrees C). Bianterior zooids could only be induced from 3-day double-anterior buds, suggesting that the anterior determination occurs later than the posterior determination. Next, a 3-day posterior half was combined with a 1-day posterior half from a higher site. Though "high-level" tissue has a high potential for posterior differentiation, the 1-day bud half failed to form the digestive tract. This result suggests that the 3-day bud half has acquired the ability to inhibit the formation of additional posterior structures. The kinetics of posterior-forming activity of developing bud pieces was measured using a 12-hr bud piece of the highest level as a probe. The activity increased with increasing developmental time, and the increase was accompanied by posterior inhibition activity. Buds of different positional levels showed different time courses of these activities. The results of this study show that the posterior end is the dominant region of developing ascidian buds.