Vascular regeneration in a basal chordate is due to the presence of immobile, bi-functional cells

Genotype-specific Expression of Uncle Fester Suggests a Role in Allorecognition Education in a Basal Chordate

Histocompatibility is the ability to discriminate between self and non-self tissues, and has been described in species throughout the metazoa. Despite its universal presence, histocompatibility genes utilized by different phyla are unique-those found in sponges, cnidarians, ascidians, and vertebrates are not orthologous. Thus, the origins of these sophisticated recognition systems, and any potential functional commonalities between them, are not understood. We are studying histocompatibility in the botryllid ascidians, members of the chordate subphylum, Tunicata, which provide a powerful model to understand both the origins and functional aspects of this process. Histocompatibility in the botryllids occurs at the tips of an extracorporeal vasculature that come into contact when two individuals grow into proximity. If compatible, the vessels will fuse, forming a parabiosis between the two individuals. If incompatible, the two vessels will reject-an inflammatory reaction that results in melanin scar formation at the point of contact, blocking anastomosis. Compatibility is determined by a single, highly polymorphic locus called the fuhc with the following rules: individuals that share one or both fuhc alleles will fuse, while those who share neither will reject. The fuhc locus encodes at least six proteins with known roles in allorecognition. One of these genes, called uncle fester, is necessary and sufficient to initiate the rejection response. Here, we report the existence of genotype-specific expression levels of uncle fester, differing by up to eight-fold at the mRNA-level, and that these expression levels are constant and maintained for the lifetime of an individual. We also found that these differences had functional consequences: the expression level of uncle fester correlated with the speed and severity of the rejection response. These findings support previous conclusions that uncle fester levels modulate the rejection response, and may be responsible for controlling the variation observed in the timing and intensity of the reaction. The maintenance of genotype specific expression of uncle fester is also evidence of an education process reminiscent of that which occurs in mammalian Natural Killer cells. In turn, this suggests that while histocompatibility receptors and ligands evolve via convergent evolution, they may utilize conserved intracellular machinery to interpret binding events at the cell surface.

Genotype-specific expression of uncle fester suggests a role in allorecognition education in a basal chordate

Histocompatibility is the ability to discriminate between self and non-self tissues, and has been described in species throughout the metazoa. Despite its universal presence, histocompatibility genes utilized by different phyla are unique- those found in sponges, cnidarians, ascidians and vertebrates are not orthologous. Thus, the origins of these sophisticated recognition systems, and any potential functional commonalities between them are not understood. A well-studied histocompatibility system exists in the botryllid ascidians, members of the chordate subphylum, Tunicata, and provides an opportunity to do so. Histocompatibility in the botryllids occurs at the tips of an extracorporeal vasculature that come into contact when two individuals grow into proximity. If compatible, the vessels will fuse, forming a parabiosis between the two individuals. If incompatible, the two vessels will reject- an inflammatory reaction that results in melanin scar formation at the point of contact, blocking anastomosis. Compatibility is determined by a single, highly polymorphic locus called the fuhc with the following rules: individuals that share one or both fuhc alleles will fuse, while those who share neither will reject. The fuhc locus encodes multiple proteins with roles in allorecognition, including one called uncle fester, which is necessary and sufficient to initiate the rejection response. Here we report the existence of genotype-specific expression levels of uncle fester, differing by up to 8-fold at the mRNA-level, and that these expression levels are constant and maintained for the lifetime of an individual. We also found that these differences had functional consequences: the expression level of uncle fester correlated with the speed and severity of the rejection response. These findings support previous conclusions that uncle fester levels modulate the rejection response, and may be responsible for controlling the variation observed in the timing and intensity of the reaction. The maintenance of genotype specific expression of uncle fester is also evidence of an education process reminiscent of that which occurs in mammalian Natural Killer (NK) cells. In turn, this suggests that while histocompatibility receptors and ligands evolve via convergent evolution, they may utilize conserved intracellular machinery to interpret binding events at the cell surface.

Increased collective migration correlates with germline stem cell competition in a basal chordate

Cell competition is a process that compares the relative fitness of progenitor cells, resulting in winners, which contribute further to development, and losers, which are excluded, and is likely a universal quality control process that contributes to the fitness of an individual. Cell competition also has pathological consequences, and can create super-competitor cells responsible for tumor progression. We are studying cell competition during germline regeneration in the colonial ascidian, Botryllus schlosseri. Germline regeneration is due to the presence of germline stem cells (GSCs) which have a unique property: a competitive phenotype. When GSCs from one individual are transplanted into another, the donor and recipient cells compete for germline development. Often the donor GSCs win, and completely replace the gametes of the recipient- a process called germ cell parasitism (gcp). gcp is a heritable trait, and winner and loser genotypes can be found in nature and reared in the lab. However, the molecular and cellular mechanisms underlying gcp are unknown. Using an ex vivo migration assay, we show that GSCs isolated from winner genotypes migrate faster and in larger clusters than losers, and that cluster size correlates with expression of the Notch ligand, Jagged. Both cluster size and jagged expression can be manipulated simultaneously in a genotype dependent manner: treatment of loser GSCs with hepatocyte growth factor increases both jagged expression and cluster size, while inhibitors of the MAPK pathway decrease jagged expression and cluster size in winner GSCs. Live imaging in individuals transplanted with labeled winner and loser GSCs reveal that they migrate to the niche, some as small clusters, with the winners having a slight advantage in niche occupancy. Together, this suggests that the basis of GSC competition resides in a combination in homing ability and niche occupancy, and may be controlled by differential utilization of the Notch pathway.

The Onset of Whole-Body Regeneration in Botryllus schlosseri: Morphological and Molecular Characterization

Colonial tunicates are the only chordates that regularly regenerate a fully functional whole body as part of their asexual life cycle, starting from specific epithelia and/or mesenchymal cells. In addition, in some species, whole-body regeneration (WBR) can also be triggered by extensive injuries, which deplete most of their tissues and organs and leave behind only small fragments of their body. In this manuscript, we characterized the onset of WBR in Botryllus schlosseri, one colonial tunicate long used as a laboratory model. We first analyzed the transcriptomic response to a WBR-triggering injury. Then, through morphological characterization, in vivo observations via time-lapse, vital dyes, and cell transplant assays, we started to reconstruct the dynamics of the cells triggering regeneration, highlighting an interplay between mesenchymal and epithelial cells. The dynamics described here suggest that WBR in B. schlosseri is initiated by extravascular tissue fragments derived from the injured individuals rather than particular populations of blood-borne cells, as has been described in closely related species. The morphological and molecular datasets here reported provide the background for future mechanistic studies of the WBR ontogenesis in B. schlosseri and allow to compare it with other regenerative processes occurring in other tunicate species and possibly independently evolved.

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.

Noninvasive Intravascular Microtransfusion in Colonial Tunicates

Tunicates are a diverse group of worldwide marine filter-feeders that are vertebrates' closest invertebrate relatives. Colonial tunicates are the only know chordates that have been shown to undergo whole-body regeneration (WBR). Botrylloides in particular can regenerate one fully functional adult from a minute fragment of their vascular system in as little as 10 days. This regenerative process relies on the proliferation of circulating stem cells, likely supported by the activity of some of the 11 identified types of hemocytes. To study and challenge WBR, it is thus important to have the capacity to isolate, analyze, and manipulate hemolymph in regenerating colonies. Here we present a microtransfusion technique that permits the collection of pure hemocytes, the quantification of their purity, their labeling, and reinjection into colonial tunicates. To exemplify our approach, we present in addition a protocol to analyze the isolated hemocytes using flow cytometry. Our approach is minimally invasive, does not induce lethality, and therefore allows repeated transfusion into exactly the same colony with minimal disruption to the process being studied.

Studying Regeneration in Ascidians: An Historical Overview

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

Vascular Aging in the Invertebrate Chordate, Botryllus schlosseri

Vascular diseases affect over 1 billion people worldwide and are highly prevalent among the elderly, due to a progressive deterioration of the structure of vascular cells. Most of our understanding of these age-related cellular changes comes from in vitro studies on human cell lines. Further studies of the mechanisms underlying vascular aging in vivo are needed to provide insight into the pathobiology of age-associated vascular diseases, but are difficult to carry out on vertebrate model organisms. We are studying the effects of aging on the vasculature of the invertebrate chordate, Botryllus schlosseri. This extracorporeal vascular network of Botryllus is transparent and particularly amenable to imaging and manipulation. Here we use a combination of transcriptomics, immunostaining and live-imaging, as well as in vivo pharmacological treatments and regeneration assays to show that morphological, transcriptional, and functional age-associated changes within vascular cells are key hallmarks of aging in B. schlosseri, and occur independent of genotype. We show that age-associated changes in the cytoskeleton and the extracellular matrix reshape vascular cells into a flattened and elongated form and there are major changes in the structure of the basement membrane over time. The vessels narrow, reducing blood flow, and become less responsive to stimuli inducing vascular regression. The extracorporeal vasculature is highly regenerative following injury, and while age does not affect the regeneration potential, newly regenerated vascular cells maintain the same aged phenotype, suggesting that aging of the vasculature is a result of heritable epigenetic changes.

Beyond Adult Stem Cells: Dedifferentiation as a Unifying Mechanism Underlying Regeneration in Invertebrate Deuterostomes

The diversity of regenerative phenomena seen in adult metazoans, as well as their underlying mechanistic bases, are still far from being comprehensively understood. Reviewing both ultrastructural and molecular data, the present work aims to showcase the increasing relevance of invertebrate deuterostomes, i.e., echinoderms, hemichordates, cephalochordates and tunicates, as invaluable models to study cellular aspects of adult regeneration. Our comparative approach suggests a fundamental contribution of local dedifferentiation -rather than mobilization of resident undifferentiated stem cells- as an important cellular mechanism contributing to regeneration in these groups. Thus, elucidating the cellular origins, recruitment and fate of cells, as well as the molecular signals underpinning tissue regrowth in regeneration-competent deuterostomes, will provide the foundation for future research in tackling the relatively limited regenerative abilities of vertebrates, with clear applications in regenerative medicine.

Characterizing the cellular architecture of dynamically remodeling vascular tissue using 3-D image analysis and virtual reconstruction

Epithelial tubules form critical structures in lung, kidney, and vascular tissues. However, the processes that control their morphogenesis and physiological expansion and contraction are not well understood. Here we examine the dynamic remodeling of epithelial tubes in vivo using a novel model system: the extracorporeal vasculature of Botryllus schlosseri, in which the disruption of the basement membrane triggers rapid, massive vascular retraction without loss of barrier function. We developed and implemented 3-D image analysis and virtual reconstruction tools to characterize the cellular morphology of the vascular wall in unmanipulated vessels and during retraction. In both control and regressed conditions, cells within the vascular wall were planar polarized, with an integrin- and curvature-dependent axial elongation of cells and a robust circumferential alignment of actin bundles. Surprisingly, we found no measurable differences in morphology between normal and retracting vessels under extracellular matrix (ECM) disruption. However, inhibition of integrin signaling through focal adhesion kinase inhibition caused disruption of cellular actin organization. Our results demonstrate that epithelial tubes can maintain tissue organization even during extreme remodeling events, but that the robust response to mechanical signals—such as the response to loss of vascular tension after ECM disruption—requires functional force sensing machinery via integrin signaling.

The biology of the extracorporeal vasculature of Botryllus schlosseri

The extracorporeal vasculature of the colonial ascidian Botryllus schlosseri plays a key role in several biological processes: transporting blood, angiogenesis, regeneration, self-nonself recognition, and parabiosis. The vasculature also interconnects all individuals in a colony and is composed of a single layer of ectodermally-derived cells. These cells form a tube with the basal lamina facing the lumen, and the apical side facing an extracellular matrix that consists of cellulose and other proteins, known as the tunic. Vascular tissue is transparent and can cover several square centimeters, which is much larger than any single individual within the colony. It forms a network that ramifies and expands to the perimeter of each colony and terminates into oval-shaped protrusions known as ampullae. Botryllus individuals replace themselves through a weekly budding cycle, and vasculature is added to ensure the interconnection of each new individual, thus there is continuous angiogenesis occurring naturally. The vascular tissue itself is highly regenerative; surgical removal of the ampullae and peripheral vasculature triggers regrowth within 24-48 h, which includes forming new ampullae. When two individuals, whether in the wild or in the lab, come into close contact and their ampullae touch, they can either undergo parabiosis through anastomosing vessels, or reject vascular fusion. The vasculature is easily manipulated by direct means such as microinjections, microsurgeries, and pharmacological reagents. Its transparent nature allows for in vivo analysis by bright field and fluorescence microscopy. Here we review the techniques and approaches developed to study the different biological processes that involve the extracorporeal vasculature.

Complex mammalian-like haematopoietic system found in a colonial chordate

Haematopoiesis is an essential process that evolved in multicellular animals. At the heart of this process are haematopoietic stem cells (HSCs), which are multipotent and self-renewing, and generate the entire repertoire of blood and immune cells throughout an animal's life1. Although there have been comprehensive studies on self-renewal, differentiation, physiological regulation and niche occupation in vertebrate HSCs, relatively little is known about the evolutionary origin and niches of these cells. Here we describe the haematopoietic system of Botryllus schlosseri, a colonial tunicate that has a vasculature and circulating blood cells, and interesting stem-cell biology and immunity characteristics2-8. Self-recognition between genetically compatible B. schlosseri colonies leads to the formation of natural parabionts with shared circulation, whereas incompatible colonies reject each other3,4,7. Using flow cytometry, whole-transcriptome sequencing of defined cell populations and diverse functional assays, we identify HSCs, progenitors, immune effector cells and an HSC niche, and demonstrate that self-recognition inhibits allospecific cytotoxic reactions. Our results show that HSC and myeloid lineage immune cells emerged in a common ancestor of tunicates and vertebrates, and also suggest that haematopoietic bone marrow and the B. schlosseri endostyle niche evolved from a common origin.

Wound-induced cell proliferation during animal regeneration

Many animal species are capable of replacing missing tissues that are lost upon injury or amputation through the process of regeneration. Although the extent of regeneration is variable across animals, that is, some animals can regenerate any missing cell type whereas some can only regenerate certain organs or tissues, regulated cell proliferation underlies the formation of new tissues in most systems. Notably, many species display an increase in proliferation within hours or days upon wounding. While different cell types proliferate in response to wounding in various animal taxa, comparative molecular data are beginning to point to shared wound-induced mechanisms that regulate cell division during regeneration. Here, we synthesize current insights about early molecular pathways of regeneration from diverse model and emerging systems by considering these species in their evolutionary contexts. Despite the great diversity of mechanisms underlying injury-induced cell proliferation across animals, and sometimes even in the same species, similar pathways for proliferation have been implicated in distantly related species (e.g., small diffusible molecules, signaling from apoptotic cells, growth factor signaling, mTOR and Hippo signaling, and Wnt and Bmp pathways). Studies that explicitly interrogate molecular and cellular regenerative mechanisms in understudied animal phyla will reveal the extent to which early pathways in the process of regeneration are conserved or independently evolved. This article is categorized under: Comparative Development and Evolution > Body Plan Evolution Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration Comparative Development and Evolution > Model Systems.

Gonad development and hermaphroditism in the ascidian Botryllus schlosseri

The colonial ascidian Botryllus schlosseri is an ideal model organism for studying gonad development and hermaphroditism. B. schlosseri has been reared in laboratories for over half a century, and its unique biology allows investigators to probe the processes of germ cell migration and gonad formation, resorption, and regeneration. Following metamorphosis, colonies of B. schlosseri show a synchronized and sequential fertility program that, under standard laboratory conditions, begins with a juvenile stage with no visible gonads and subsequently develops testes at 9 weeks followed later by the production of oocytes-thus resulting in hermaphroditic individuals. The timing of oocyte production varies according to the season, and adult B. schlosseri colonies can cycle among infertile and both male and hermaphrodite fertile states in response to changing environmental conditions. Thus, these acidians are amenable to studying the molecular mechanisms controlling fertility, and recent genomic and transcriptomic databases are providing insight to the key genes involved. Here, we review the techniques and approaches developed to study germ cell migration and gonad formation in B. schlosseri, and include novel videos showing processes related to oocyte ovulation and sperm discharge. In the future, this valuable invertebrate model system may help understand the mechanisms of gonad development and regeneration in a chordate. Mol. Reprod. Dev. 84: 158-170, 2017. © 2016 Wiley Periodicals, Inc.

In vivo manipulation of the extracellular matrix induces vascular regression in a basal chordate

We investigated the physical role of the extracellular matrix (ECM) in vascular homeostasis in the basal chordate Botryllus schlosseri, which has a large, transparent, extracorporeal vascular network encompassing an area >100 cm² We found that the collagen cross-linking enzyme lysyl oxidase is expressed in all vascular cells and that in vivo inhibition using β-aminopropionitrile (BAPN) caused a rapid, global regression of the entire network, with some vessels regressing >10 mm within 16 h. BAPN treatment changed the ultrastructure of collagen fibers in the vessel basement membrane, and the kinetics of regression were dose dependent. Pharmacological inhibition of both focal adhesion kinase (FAK) and Raf also induced regression, and levels of phosphorylated FAK in vascular cells decreased during BAPN treatment and FAK inhibition but not Raf inhibition, suggesting that physical changes in the vessel ECM are detected via canonical integrin signaling pathways. Regression is driven by apoptosis and extrusion of cells through the basal lamina, which are then engulfed by blood-borne phagocytes. Extrusion and regression occurred in a coordinated manner that maintained vessel integrity, with no loss of barrier function. This suggests the presence of regulatory mechanisms linking physical changes to a homeostatic, tissue-level response.

Colonial ascidians as model organisms for the study of germ cells, fertility, whole body regeneration, vascular biology and aging

Colonial ascidians are marine invertebrate chordates that are the closest invertebrate relative to the vertebrates. Colonies of Botryllus schlosseri undergo a continuous asexual reproduction process that involves the regeneration of entire new bodies, which include all somatic and germline tissues. This adult regenerative process depends on signaling pathways known to regulate the formation of tissues and organs during embryonic development. The formation of gonads within regenerated bodies depends on migration and homing of germ cell precursors to niches within the developing bodies, and Botryllus colonies can cycle between fertile and infertile states. The vasculature that connects all individuals within the colony is highly regenerative, and is a valuable tool for the study of angiogenesis in adult blood vessels. The tremendous regenerative capacity of the vasculature even results in regeneration of entire new bodies solely from fragments of blood vessels upon surgical removal of all bodies. The mechanism underlying this regeneration of whole bodies is not well understood, but appears to depend on proliferation of circulating, blood borne cells. Because of all of these features, colonial ascidians are ideal model organisms for the study of germ cell migration, fertility, vascular biology and regeneration.

Transcriptome dynamics in the asexual cycle of the chordate Botryllus schlosseri

Background

We performed an analysis of the transcriptome during the blastogenesis of the chordate Botryllus schlosseri, focusing in particular on genes involved in cell death by apoptosis. The tunicate B. schlosseri is an ascidian forming colonies characterized by the coexistence of three blastogenetic generations: filter-feeding adults, buds on adults, and budlets on buds. Cyclically, adult tissues undergo apoptosis and are progressively resorbed and replaced by their buds originated by asexual reproduction. This is a feature of colonial tunicates, the only known chordates that can reproduce asexually.

Results

Thanks to a newly developed web-based platform (http://botryllus.cribi.unipd.it), we compared the transcriptomes of the mid-cycle, the pre-take-over, and the take-over phases of the colonial blastogenetic cycle. The platform is equipped with programs for comparative analysis and allows to select the statistical stringency. We enriched the genome annotation with 11,337 new genes; 581 transcripts were resolved as complete open reading frames, translated in silico into amino acid sequences and then aligned onto the non-redundant sequence database. Significant differentially expressed genes were classified within the gene ontology categories. Among them, we recognized genes involved in apoptosis activation, de-activation, and regulation.

Conclusions

With the current work, we contributed to the improvement of the first released B. schlosseri genome assembly and offer an overview of the transcriptome changes during the blastogenetic cycle, showing up- and down-regulated genes. These results are important for the comprehension of the events underlying colony growth and regression, cell proliferation, colony homeostasis, and competition among different generations.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2598-1) contains supplementary material, which is available to authorized users.

Molecular evolution and in vitro characterization of Botryllus histocompatibility factor

Botryllus schlosseri is a colonial ascidian with a natural ability to anastomose with another colony to form a vascular and hematopoietic chimera. In order to fuse, two individuals must share at least one allele at the highly polymorphic fuhc locus. Otherwise, a blood-based inflammatory response will occur resulting in a melanin scar at the sites of interaction. The single-locus genetic control of allorecognition makes B. schlosseri an attractive model to study the underlying molecular mechanisms. Over the past decade, several candidate genes involved in allorecognition have been identified, but how they ultimately contribute to allorecognition outcome remains poorly understood. Here, we report our initial molecular characterization of a recently identified candidate allodeterminant called Botryllus histocompatibility factor (bhf). bhf, both on a DNA and protein level, is the least polymorphic protein in the fuhc locus studied so far and, unlike other known allorecognition determinants, does not appear to be under any form of balancing or directional selection. Additionally, we identified a second isoform through mRNA-Seq and an EST assembly library which is missing exon 3, resulting in a C-terminally truncated form. We report via whole-mount fluorescent in situ hybridization that a subset of cells co-express bhf and cfuhc(sec). Finally, we observed BHF's localization in HEK293T at the cytoplasmic side of the plasma membrane in addition to the nucleus via a nuclear localization signal. Given the localization data thus far, we hypothesize that BHF may function as a scaffolding protein in a complex with other Botryllus proteins, rather than functioning as an allorecognition determinant.

Aging in the colonial chordate, Botryllus schlosseri

What mechanisms underlie aging? One theory, the wear-and-tear model, attributes aging to progressive deterioration in the molecular and cellular machinery which eventually lead to death through the disruption of physiological homeostasis. The second suggests that life span is genetically programmed, and aging may be derived from intrinsic processes which enforce a non-random, terminal time interval for the survivability of the organism. We are studying an organism that demonstrates both properties: the colonial ascidian, Botryllus schlosseri. Botryllus is a member of the Tunicata, the sister group to the vertebrates, and has a number of life history traits which make it an excellent model for studies on aging. First, Botryllus has a colonial life history, and grows by a process of asexual reproduction during which entire bodies, including all somatic and germline lineages, regenerate every week, resulting in a colony of genetically identical individuals. Second, previous studies of lifespan in genetically distinct Botryllus lineages suggest that a direct, heritable basis underlying mortality exists that is unlinked to reproductive effort and other life history traits. Here we will review recent efforts to take advantage of the unique life history traits of B. schlosseri and develop it into a robust model for aging research.

Invertebrates as model organisms for research on aging biology

Invertebrate model systems, such as nematodes and fruit flies, have provided valuable information about the genetics and cellular biology involved in aging. However, limitations of these simple, genetically tractable organisms suggest the need for other model systems, some of them invertebrate, to facilitate further advances in the understanding of mechanisms of aging and longevity in mammals, including humans. This paper introduces 10 review articles about the use of invertebrate model systems for the study of aging by authors who participated in an 'NIA-NIH symposium on aging in invertebrate model systems' at the 2013 International Congress for Invertebrate Reproduction and Development. In contrast to the highly derived characteristics of nematodes and fruit flies as members of the superphylum Ecdysozoa, cnidarians, such as Hydra, are more 'basal' organisms that have a greater number of genetic orthologs in common with humans. Moreover, some other new model systems, such as the urochordate Botryllus schlosseri, the tunicate Ciona, and the sea urchins (Echinodermata) are members of the Deuterostomia, the same superphylum that includes all vertebrates, and thus have mechanisms that are likely to be more closely related to those occurring in humans. Additional characteristics of these new model systems, such as the recent development of new molecular and genetic tools and a more similar pattern to humans of regeneration and stem cell function suggest that these new model systems may have unique advantages for the study of mechanisms of aging and longevity.

Testing an unusual in vivo vessel network model: a method to study angiogenesis in the colonial tunicate Botryllus schlosseri

Tunicates are the closest relatives to vertebrates and include the only chordate species able to reproduce both sexually and asexually. The colonial tunicate Botryllus schlosseri is embedded in a transparent extracellular matrix (the tunic) containing the colonial circulatory system (CCS). The latter is a network of vessels external to zooids, limited by a simple, flat epithelium that originated from the epidermis. The CCS propagates and regenerates by remodelling and extending the vessel network through the mechanism of sprouting, which typically characterises vertebrate angiogenesis. In exploiting the characteristics of B. schlosseri as a laboratory model, we present a new experimental and analysis method based on the ability to obtain genetically identical subclones representing paired samples for the appropriate quantitative outcome statistical analysis. The method, tested using human VEGF and EGF to induce angiogenesis, shows that the CCS provides a useful in vivo vessel network model for testing the effects of specific injected solutes on vessel dynamics. These results show the potentiality of B. schlosseri CCS as an effective complementary model for in vivo studies on angiogenesis and anticancer therapy. We discuss this potentiality, taking into consideration the origin, nature, and roles of the cellular and molecular agents involved in CCS growth.