A conserved role of the VEGF pathway in angiogenesis of an ectodermally-derived vasculature

Expansion and collapse of VEGF diversity in major clades of the animal kingdom

Together with the platelet-derived growth factors (PDGFs), the vascular endothelial growth factors (VEGFs) form the PDGF/VEGF subgroup among cystine knot growth factors. The evolutionary relationships within this subgroup have not been examined thoroughly to date. Here, we comprehensively analyze the PDGF/VEGF growth factors throughout all animal phyla and propose a phylogenetic tree. Vertebrate whole-genome duplications play a role in expanding PDGF/VEGF diversity, but several limited duplications are necessary to account for the temporal pattern of emergence. The phylogenetically oldest PDGF/VEGF-like growth factor likely featured a C-terminus with a BR3P signature, a hallmark of the modern-day lymphangiogenic growth factors VEGF-C and VEGF-D. Some younger VEGF genes, such as VEGFB and PGF, appeared completely absent in important vertebrate clades such as birds and amphibia, respectively. In contrast, individual PDGF/VEGF gene duplications frequently occurred in fish on top of the known fish-specific whole-genome duplications. The lack of precise counterparts for human genes poses limitations but also offers opportunities for research using organisms that diverge considerably from humans. Sources for the graphical abstract: 326 MYA and older [1]; 72-240 MYA [2]; 235-65 MYA [3].

Targeting the tumor microenvironment: Potential strategy for cancer therapeutics

Cellular and stromal components including tumor cells, immune cells, mesenchymal cells, cancer-linked fibroblasts, and extracellular matrix, constituent tumor microenvironment (TME). TME plays a crucial role in reprogramming tumor initiation, uncontrolled proliferation, invasion and metastasis as well as response to therapeutic modalities. In recent years targeting the TME has developed as a potential strategy for treatment of cancer because of its life-threatening functions in restricting tumor development and modulating responses to standard-of-care medicines. Cold atmospheric plasma, oncolytic viral therapy, bacterial therapy, nano-vaccine, and repurposed pharmaceuticals with combination therapy, antiangiogenic drugs, and immunotherapies are among the most effective therapies directed by TME that have either been clinically authorized or are currently being studied. This article discusses above-mentioned therapies in light of targeting TME. We also cover problems related to the TME-targeted therapies, as well as future insights and practical uses in this rapidly growing field.

Distinct regulatory states control the elongation of individual skeletal rods in the sea urchin embryo

Background

Understanding how gene regulatory networks (GRNs) control developmental progression is a key to the mechanistic understanding of morphogenesis. The sea urchin larval skeletogenesis provides an excellent platform to tackle this question. In the early stages of sea urchin skeletogenesis, skeletogenic genes are uniformly expressed in the skeletogenic lineage. Yet, during skeletal elongation, skeletogenic genes are expressed in distinct spatial sub‐domains. The regulation of differential gene expression during late skeletogenesis is not well understood.

Results

Here we reveal the dynamic expression of the skeletogenic regulatory genes that define a specific regulatory state for each pair of skeletal rods, in the sea urchin Paracentrotus lividus. The vascular endothelial growth factor (VEGF) signaling, essential for skeleton formation, specifically controls the migration of cells that form the postoral and distal anterolateral skeletogenic rods. VEGF signaling also controls the expression of regulatory genes in cells at the tips of the postoral rods, including the transcription factors Pitx1 and MyoD1. Pitx1 activity is required for normal skeletal elongation and for the expression of some of VEGF target genes.

Conclusions

Our study illuminates the fine‐tuning of the regulatory system during the transition from early to late skeletogenesis that gives rise to rod‐specific regulatory states.

The skeletogenic transcription factors form specific regulatory states in various skeletogenic sub‐populations.

Late VEGF signaling controls the regulatory states at the tips of the post‐oral and anterolateral skeletal rods.

VEGF signaling controls the expression of the transcription factors, MyoD1 and Pitx1.

Pitx1 activity is required for normal skeletal elongation and for the expression of some of VEGF target genes.

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.

The Evolution of Biomineralization through the Co-Option of Organic Scaffold Forming Networks

Biomineralization is the process in which organisms use minerals to generate hard structures like teeth, skeletons and shells. Biomineralization is proposed to have evolved independently in different phyla through the co-option of pre-existing developmental programs. Comparing the gene regulatory networks (GRNs) that drive biomineralization in different species could illuminate the molecular evolution of biomineralization. Skeletogenesis in the sea urchin embryo was extensively studied and the underlying GRN shows high conservation within echinoderms, larval and adult skeletogenesis. The organic scaffold in which the calcite skeletal elements form in echinoderms is a tubular compartment generated by the syncytial skeletogenic cells. This is strictly different than the organic cartilaginous scaffold that vertebrates mineralize with hydroxyapatite to make their bones. Here I compare the GRNs that drive biomineralization and tubulogenesis in echinoderms and in vertebrates. The GRN that drives skeletogenesis in the sea urchin embryo shows little similarity to the GRN that drives bone formation and high resemblance to the GRN that drives vertebrates’ vascular tubulogenesis. On the other hand, vertebrates’ bone-GRNs show high similarity to the GRNs that operate in the cells that generate the cartilage-like tissues of basal chordate and invertebrates that do not produce mineralized tissue. These comparisons suggest that biomineralization in deuterostomes evolved through the phylum specific co-option of GRNs that control distinct organic scaffolds to mineralization.

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.

The tolerance to hypoxia is defined by a time-sensitive response of the gene regulatory network in sea urchin embryos

Deoxygenation, the reduction of oxygen level in the oceans induced by global warming and anthropogenic disturbances, is a major threat to marine life. This change in oxygen level could be especially harmful to marine embryos that use endogenous hypoxia and redox gradients as morphogens during normal development. Here, we show that the tolerance to hypoxic conditions changes between different developmental stages of the sea urchin embryo, possibly due to the structure of the gene regulatory networks (GRNs). We demonstrate that during normal development, the bone morphogenetic protein (BMP) pathway restricts the activity of the vascular endothelial growth factor (VEGF) pathway to two lateral domains and this restriction controls proper skeletal patterning. Hypoxia applied during early development strongly perturbs the activity of Nodal and BMP pathways that affect the VEGF pathway, dorsal-ventral (DV) and skeletogenic patterning. These pathways are largely unaffected by hypoxia applied after DV-axis formation. We propose that the use of redox and hypoxia as morphogens makes the sea urchin embryo highly sensitive to environmental hypoxia during early development, but the GRN structure provides higher tolerance to hypoxia at later stages.

Summary: The use of hypoxia and redox gradients as morphogens makes sea urchin early development sensitive to environmental hypoxia. This sensitivity decreases later, possibly due to the gene regulatory network structure.

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.

Cell-Matrix Interactions in the Eye: From Cornea to Choroid

The extracellular matrix (ECM) plays a crucial role in all parts of the eye, from maintaining clarity and hydration of the cornea and vitreous to regulating angiogenesis, intraocular pressure maintenance, and vascular signaling. This review focuses on the interactions of the ECM for homeostasis of normal physiologic functions of the cornea, vitreous, retina, retinal pigment epithelium, Bruch's membrane, and choroid as well as trabecular meshwork, optic nerve, conjunctiva and tenon's layer as it relates to glaucoma. A variety of pathways and key factors related to ECM in the eye are discussed, including but not limited to those related to transforming growth factor-β, vascular endothelial growth factor, basic-fibroblastic growth factor, connective tissue growth factor, matrix metalloproteinases (including MMP-2 and MMP-9, and MMP-14), collagen IV, fibronectin, elastin, canonical signaling, integrins, and endothelial morphogenesis consistent of cellular activation-tubulogenesis and cellular differentiation-stabilization. Alterations contributing to disease states such as wound healing, diabetes-related complications, Fuchs endothelial corneal dystrophy, angiogenesis, fibrosis, age-related macular degeneration, retinal detachment, and posteriorly inserted vitreous base are also reviewed.

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.

Genes with evidence of positive selection as potentially related to coloniality and the evolution of morphological features among the lophophorates and entoprocts

Evolutionary mechanisms that underlie the origins of coloniality among organisms are diverse. Some animal colonies may be comprised strictly of clonal individuals formed from asexual budding or comprised of a chimera of clonal and sexually produced individuals that fuse secondarily. This investigation focuses on select members of the lophophorates and entoprocts whose evolutionary relationships remain enigmatic even in the age of genomics. Using transcriptomic data sets, two coloniality-based hypotheses are tested in a phylogenetic context to find candidate genes showing evidence of positive selection and potentially convergent molecular signatures among solitary species and taxa-forming colonies from aggregate groups or clonal budding. Approximately 22% of the 387 orthogroups tested showed evidence of positive selection in at least one of the three branch-site tests (CODEML, BUSTED, and aBSREL). Only 12 genes could be reliably associated with a developmental function related to traits linked with coloniality, neuroanatomy, or ciliary fields. Genes testing for both positive selection and convergent molecular characters include orthologues of Radial spoke head, Elongation translation initiation factors, SEC13, and Immediate early response gene5. Maximum likelihood analyses included here resulted in tree topologies typical of other phylogenetic investigations based on wider genomic information. Further genomic and experimental evidence will be needed to resolve whether a solitary ancestor with multiciliated cells that formed aggregate groups gave rise to colonial forms in bryozoans (and perhaps the entoprocts) or that the morphological differences exhibited by phoronids and brachiopods represent trait modifications from a colonial ancestor.

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.

Possible cooption of a VEGF-driven tubulogenesis program for biomineralization in echinoderms

Biomineralization is the process by which living organisms use minerals to form hard structures that protect and support them. Biomineralization is believed to have evolved rapidly and independently in different phyla utilizing preexisting components. The mechanistic understanding of the regulatory networks that drive biomineralization and their evolution is far from clear. Sea urchin skeletogenesis is an excellent model system for studying both gene regulation and mineral uptake and deposition. The sea urchin calcite spicules are formed within a tubular cavity generated by the skeletogenic cells controlled by vascular endothelial growth factor (VEGF) signaling. The VEGF pathway is essential for biomineralization in echinoderms, while in many other phyla, across metazoans, it controls tubulogenesis and vascularization. Despite the critical role of VEGF signaling in sea urchin spiculogenesis, the downstream program it activates was largely unknown. Here we study the cellular and molecular machinery activated by the VEGF pathway during sea urchin spiculogenesis and reveal multiple parallels to the regulation of vertebrate vascularization. Human VEGF rescues sea urchin VEGF knockdown, vesicle deposition into an internal cavity plays a significant role in both systems, and sea urchin VEGF signaling activates hundreds of genes, including biomineralization and interestingly, vascularization genes. Moreover, five upstream transcription factors and three signaling genes that drive spiculogenesis are homologous to vertebrate factors that control vascularization. Overall, our findings suggest that sea urchin spiculogenesis and vertebrate vascularization diverged from a common ancestral tubulogenesis program, broadly adapted for vascularization and specifically coopted for biomineralization in the echinoderm phylum.

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.

Whole-Body Regeneration in the Colonial Tunicate Botrylloides leachii

The colonial marine invertebrate Botrylloides leachii belongs to the Tunicata subphylum, the closest invertebrate relatives to the vertebrate group and the only known class of chordates that can undergo whole-body regeneration (WBR). This dramatic developmental process allows a minute isolated fragment of B. leachii's vascular system, or a colony excised of all adults, to restore a functional animal in as little as 10 days. In addition to this exceptional regenerative capacity, B. leachii can reproduce both sexually, through a tadpole larval stage, and asexually, through palleal budding. Thus, three alternative developmental strategies lead to the establishment of filter-feeding adults. Consequently, B. leachii is particularly well suited for comparative studies on regeneration and should provide novel insights into regenerative processes in chordates.Here, after a short introduction on regeneration, we overview the biology of B. leachii as well as the current state of knowledge on WBR in this species and in related species of tunicates. Finally, we highlight the possible future directions that research might take in the study of WBR, including thoughts on technological approaches that appear most promising in this context. Overall, we provide a synthesis of the current knowledge on WBR in B. leachii to support research in this chordate species.

Inhibition of Vascular Endothelial Growth Factor Receptor Decreases Regenerative Angiogenesis in Axolotls

Angiogenesis is crucial for tissue growth and repair in mammals, and is chiefly regulated by vascular endothelial growth factor (VEGF) signaling. We evaluated the effect of chemical inhibition of VEGF receptor signaling in animals with superior regenerative ability, axolotl salamanders, to determine the impact on vascularization and regenerative outgrowth. Following tail amputation, treated animals (100 nM PTK787) and controls were examined microscopically and measured over the month-long period of regeneration. Treatment with VEGFR inhibitor decreased regenerative angiogenesis; drug-treated animals had lower vascular densities in the regenerating tail than untreated animals. This decrease in neovascularization, however, was not associated with a decrease in regenerative outgrowth or with morphological abnormalities in the regrown tail. Avascular but otherwise anatomically normal regenerative outgrowth over 1 mm beyond the amputation plane was observed. The results suggest that in this highly regenerative species, significant early tissue regeneration is possible in the absence of a well-developed vasculature. This research sets the groundwork for establishing a system for the chemical manipulation of angiogenesis within the highly regenerative axolotl model, contributing to a better understanding of the role of the microvasculature within strongly proliferative yet well-regulated environments. Anat Rec, 300:2273-2280, 2017. © 2017 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.

Identification of differentially expressed genes from multipotent epithelia at the onset of an asexual development

Organisms that have evolved alternative modes of reproduction, complementary to the sexual mode, are found across metazoans. The chordate Botryllus schlosseri is an emerging model for asexual development studies. Botryllus can rebuild its entire body from a portion of adult epithelia in a continuous and stereotyped process called blastogenesis. Anatomy and ontogenies of blastogenesis are well described, however molecular signatures triggering this developmental process are entirely unknown. We isolated tissues at the site of blastogenesis onset and from the same epithelia where this process is never triggered. We linearly amplified an ultra-low amount of mRNA (<10ng) and generated three transcriptome datasets. To provide a conservative landscape of transcripts differentially expressed between blastogenic vs. non-blastogenic epithelia we compared three different mapping and analysis strategies with a de novo assembled transcriptome and partially assembled genome as references, additionally a self-mapping strategy on the dataset. A subset of differentially expressed genes were analyzed and validated by in situ hybridization. The comparison of different analyses allowed us to isolate stringent sets of target genes, including transcripts with potential involvement in the onset of a non-embryonic developmental pathway. The results provide a good entry point to approach regenerative event in a basal chordate.

Redeployment of germ layers related TFs shows regionalized expression during two non-embryonic developments

In all non-vertebrate metazoan phyla, species that evolved non-embryonic developmental pathways as means of propagation or regeneration can be found. In this context, new bodies arise through asexual reproduction processes (such as budding) or whole body regeneration, that lack the familiar temporal and spatial cues classically associated with embryogenesis, like maternal determinants, or gastrulation. The molecular mechanisms underlying those non-embryonic developments (i.e., regeneration and asexual reproduction), and their relationship to those deployed during embryogenesis are poorly understood. We have addressed this question in the colonial ascidian Botryllus schlosseri, which undergoes an asexual reproductive process via palleal budding (PB), as well as a whole body regeneration by vascular budding (VB). We identified early regenerative structures during VB and then followed the fate of differentiating tissues during both non-embryonic developments (PB and VB) by monitoring the expression of genes known to play key functions in germ layer specification with well conserved expression patterns in solitary ascidian embryogenesis. The expression patterns of FoxA1, GATAa, GATAb, Otx, Bra, Gsc and Tbx2/3 were analysed during both PB and VB. We found that the majority of these transcription factors were expressed during both non-embryonic developmental processes, revealing a regionalization of the palleal and vascular buds. Knockdown of GATAa by siRNA in palleal buds confirmed that preventing the correct development of one of these regions blocks further tissue specification. Our results indicate that during both normal and injury-induced budding, a similar alternative developmental program operates via early commitment of epithelial regions.

Vascular endothelial growth factors: A comparison between invertebrates and vertebrates

This review aims to summarize recent data concerning the structure and role of the members of the vascular endothelial growth factor (VEGF) and vascular endothelial growth factor receptor (VEGFR) families in the context of early development, organogenesis and regeneration, with a particular emphasis on the role of these factors in the development of invertebrates. Homologs of VEGF and/or VEGFR have been found in all Eumetazoa, in both Radiata and Bilateria, where they are expressed in the descendants of different germ layers and play a pivotal role in the development of animals with and without a vascular system. VEGF is a well-known angiogenesis regulator, but this factor also control cell migration during neurogenesis and the development of branching organs (the trachea) in invertebrate and vertebrate species. A possible explanation for the origin of Vegf/Vegfr in the animal kingdom and a pathway of Vegf/Vegfr evolution are discussed.

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.

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.

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

The source of tissue turnover during homeostasis or following injury is usually due to proliferation of a small number of resident, lineage-restricted stem cells that have the ability to amplify and differentiate into mature cell types. We are studying vascular regeneration in a chordate model organism, Botryllus schlosseri, and have previously found that following surgical ablation of the extracorporeal vasculature, new tissue will regenerate in a VEGF-dependent process within 48 hrs. Here we use a novel vascular cell lineage tracing methodology to assess regeneration in parabiosed individuals and demonstrate that the source of regenerated vasculature is due to the proliferation of pre-existing vascular resident cells and not a mobile progenitor. We also show that these cells are bi-potential, and can reversibly adopt two fates, that of the newly forming vessels or the differentiated vascular tissue at the terminus of the vasculature, known as ampullae. In addition, we show that pre-existing vascular resident cells differentially express progenitor and differentiated cell markers including the Botryllus homologs of CD133, VEGFR-2, and Cadherin during the regenerative process.

Serum angiogenin level in sickle cell disease and beta thalassemia patients

INTRODUCTION

Angiogenesis has been investigated in different kinds of anemia. However, its role as a marker of angiogenesis has not been investigated in thalassemia or sickle cell disease (SCD).

OBJECTIVES

We aimed to investigate serum angiogenin level in children and adolescents with beta thalassemia or SCD and its relation to possible risk factors of angiogenesis.

MATERIALS AND METHODS

This study included; 32 β-thalassemia major (β-TM) patients aged 14.2 ± 3.8 years, 20 β-thalassemia intermedia (β-TI) patients aged 14.3 ± 4.8 years, 20 SCD patients aged 14.1 ± 2.4 years; 8 with (HbSS) and 12 with sickle thalassemia (HbS/β-thalassemia) and 35 age and sex-matched controls. Data collected regarding; age, sex, disease duration, blood transfusion frequency, transfusion index, chelation type and duration, CBC, Hb electrophoresis, serum ferritin and serum angiogenin level (by ELISA).

RESULTS

Angiogenin level was significantly higher in patients with SCD [250 (100-300) pg/mL] compared to β-TM [180 (140-230) pg/mL] and controls [89 (80-103) pg/mL] (P < .001) especially those with HbSS (P = .06). There was a significant negative correlation between serum angiogenin and age of patients, age of onset and duration of chelation in β-TM (P < .01, P < .001, P = .003) and β-TI (P = .009, P = .03, P < .001) and with serum ferritin in β-TI group (r = -0.573, P = .008). In SCD, angiogenin level was negatively correlated with both frequency of blood transfusion (r = -0.731, P < .001) and duration of hydroxyurea therapy (P = .017).

CONCLUSIONS

High angiogenin level detected among patients with SCD may be negatively influenced by regular blood transfusion and hydroxyurea therapy, while; early onset of chelation therapy may decrease angiogenin level in β-TM.

The genome sequence of the colonial chordate, Botryllus schlosseri

Botryllus schlosseri is a colonial urochordate that follows the chordate plan of development following sexual reproduction, but invokes a stem cell-mediated budding program during subsequent rounds of asexual reproduction. As urochordates are considered to be the closest living invertebrate relatives of vertebrates, they are ideal subjects for whole genome sequence analyses. Using a novel method for high-throughput sequencing of eukaryotic genomes, we sequenced and assembled 580 Mbp of the B. schlosseri genome. The genome assembly is comprised of nearly 14,000 intron-containing predicted genes, and 13,500 intron-less predicted genes, 40% of which could be confidently parceled into 13 (of 16 haploid) chromosomes. A comparison of homologous genes between B. schlosseri and other diverse taxonomic groups revealed genomic events underlying the evolution of vertebrates and lymphoid-mediated immunity. The B. schlosseri genome is a community resource for studying alternative modes of reproduction, natural transplantation reactions, and stem cell-mediated regeneration. DOI:http://dx.doi.org/10.7554/eLife.00569.001.

Vascular Endothelial Growth Factor Receptor Family in Ascidians, Halocynthia roretzi (Sea Squirt). Its High Expression in Circulatory System-Containing Tissues

The vascular endothelial growth factor (VEGF)-VEGF Receptor (VEGFR) system is an important pathway for regulation of angiogenesis. However, its evolutionary development, particularly the step from invertebrates to vertebrates, is still largely unknown. Here, we molecularly cloned the VEGFR-like gene from Halocynthia roretzi, a species belonging to the Tunicata, the chordate subphylum recently considered the sister group of vertebrates. The cDNA encoded a homolog of human VEGFR, including the transmembrane domain, and the tyrosine kinase domain with a kinase-insert region, which was designated S. sq VEGFR (GenBank AB374180). Similar to Tunicates including ascidians in the phylogenetic tree, the Amphioxus, another chordate, is located close to vertebrates. However, S. sq VEGFR has a higher homology than the Amphioxus VEGFR-like molecule (GenBank AB025557) to human VEGFR in the kinase domain-2 region. The S. sq VEGFR mRNA was expressed at highest levels in circulatory system-containing tissues, suggesting that S. sq VEGFR plays an important role in the formation or maintenance of circulatory system in Tunicates, Halocynthia roretzi.

Phagocyte dynamics in a highly regenerative urochordate: insights into development and host defense

Phagocytosis is a cellular process by which particles and foreign bodies are engulfed and degraded by specialized cells. It is functionally involved in nutrient acquisition and represents a fundamental mechanism used to remove pathogens and cellular debris. In the marine invertebrate chordate Botryllus schlosseri, cell corpse engulfment by phagocytic cells is the recurrent mechanism of programmed cell clearance and a critical process for the successful execution of asexual regeneration and colony homeostasis. In the present study, we have utilized a naturally occurring process of vascular parabiosis coupled with intravascular microinjection of fluorescent bioparticles and liposomes as tools to investigate the dynamics of phagocyte behavior in real-time during cyclical body regeneration. Our findings indicate that B. schlosseri harbors two major populations of post-mitotic phagocytes, which display distinct phagocytic specificity and homing patterns: a static population that lines the circulatory system epithelia, and a mobile population that continuously recirculates throughout the colony and exhibits a characteristic homing pattern within mesenchymal niches called ventral islands (VI). We observed that a significant proportion of ventral island phagocytes (VIP) die and are engulfed by other VIP following takeover. Selective impairment of VIP activity curtailed zooid resorption and asexual development. Together, these findings strongly suggest that ventral islands are sites of phagocyte homing and turnover. As botryllid ascidians represent invertebrate chordates capable of whole body regeneration in a non-embryonic scenario, we discuss the pivotal role that phagocytosis plays in homeostasis, tissue renewal and host defense.

Asexual propagation and regeneration in colonial ascidians

Regeneration is widely distributed among the metazoans. However, clear differences exist as to the degree of regenerative capacity: some phyla can only replace missing body parts, whereas others can generate entirely new individuals. Ascidians are animals that possess a remarkable regenerative plasticity and exhibit a great diversity of mechanisms for asexual propagation and survival. They are marine invertebrate members of the subphylum Tunicata and represent modern-day descendants of the chordate ancestor; in their tadpole stage they exhibit a chordate body plan that is resorbed during metamorphosis. Solitary species grow into an adult that can reach several centimeters in length, whereas colonial species grow by asexual propagation, creating a colony of genetically identical individuals. In this review, we present an overview of the biology of colonial ascidians as a paradigm for study in stem cell and regenerative biology. Focusing on botryllid ascidians, we introduce the potential roles played by multipotent epithelia and multipotent/pluripotent stem cells as source of asexual propagation and regenerative plasticity in the different budding mechanisms, and consider the putative mechanism of body repatterning in a non-embryonic scenario. We also discuss the involvement of intra-colony homeostatic processes in regulating budding potential, and the functional link between allorecognition, chimerism, and regenerative potential.

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.

Evolution of the VEGF-regulated vascular network from a neural guidance system

The vascular network is closely linked to the neural system, and an interdependence is displayed in healthy and in pathophysiological responses. How has close apposition of two such functionally different systems occurred? Here, we present a hypothesis for the evolution of the vascular network from an ancestral neural guidance system. Biological cornerstones of this hypothesis are the vascular endothelial growth factor (VEGF) protein family and cognate receptors. The primary sequences of such proteins are conserved from invertebrates, such as worms and flies that lack discernible vascular systems compared to mammals, but all these systems have sophisticated neuronal wiring involving such molecules. Ancestral VEGFs and receptors (VEGFRs) could have been used to develop and maintain the nervous system in primitive eukaryotes. During evolution, the demands of increased morphological complexity required systems for transporting molecules and cells, i.e., biological conductive tubes. We propose that the VEGF-VEGFR axis was subverted by evolution to mediate the formation of biological tubes necessary for transport of fluids, e.g., blood. Increasingly, there is evidence that aberrant VEGF-mediated responses are also linked to neuronal dysfunctions ranging from motor neuron disease, stroke, Parkinson's disease, Alzheimer's disease, ischemic brain disease, epilepsy, multiple sclerosis, and neuronal repair after injury, as well as common vascular diseases (e.g., retinal disease). Manipulation and correction of the VEGF response in different neural tissues could be an effective strategy to treat different neurological diseases.

Embryological methods in ascidians: the Villefranche-sur-Mer protocols

Ascidians (marine invertebrates: urochordates) are thought to be the closest sister groups of vertebrates. They are particularly attractive models because of their non-duplicated genome and the fast and synchronous development of large populations of eggs into simple tadpoles made of about 3,000 cells. As a result of stereotyped asymmetric cleavage patterns all blastomeres become fate restricted between the 16- and 110 cell stage through inheritance of maternal determinants and/or cellular interactions. These advantageous features have allowed advances in our understanding of the nature and role of maternal determinants, inductive interactions, and gene networks that are involved in cell lineage specification and differentiation of embryonic tissues. Ascidians have also contributed to our understanding of fertilization, cell cycle control, self-recognition, metamorphosis, and regeneration. In this chapter we provide basic protocols routinely used at the marine station in Villefranche-sur-Mer using the cosmopolitan species of reference Ciona intestinalis and the European species Phallusia mammillata. These two models present complementary advantages with regard to molecular, functional, and imaging approaches. We describe techniques for basic culture of embryos, micro-injection, in vivo labelling, micro-manipulations, fixation, and immuno-labelling. These methods allow analysis of calcium signals, reorganizations of cytoplasmic and cortical domains, meiotic and mitotic cell cycle and cleavages as well as the roles of specific genes and cellular interactions. Ascidians eggs and embryos are also an ideal material to isolate cortical fragments and to isolate and re-associate individual blastomeres. We detail the experimental manipulations which we have used to understand the structure and role of the egg cortex and of specific blastomeres during development.

Evaluation of angiogenesis with vascular endothelial growth factor in patients with thalassemia major

BACKGROUND

Thalassemia major (TM) is an important cause of severe anemia that necessitates regular blood transfusion to prevent the profound weakness and cardiac decompensation caused by the anemia. However, iron overloading is an inevitable consequence of prolonged transfusion therapy. In addition, extramedullary hematopoiesis and hemosiderosis cause spleen, liver and marrow enlargement. In recent years the role of angiogenesis has been investigated in physiological and pathological conditions. However, it is known that angiogenetic factors, especially the vascular endothelial growth factor (VEGF), cause differentiation of the hemangioblast.

METHODS

The effect of angiogenesis hasn't been investigated in TM patients yet, and in this study, angiogenesis was researched in 43 thalassemic patients by serum VEGF measurement.

RESULTS

VEGF levels were not affected by hemoglobin levels, ferritin levels, or chelation type (P > 0.05). However, VEGF was positively affected by chelation starting age and negatively affected by yearly transfusion requirement of TM patients (P < 0.05). In addition, VEGF of patients who underwent splenectomy were higher than those who didn't undergo splenectomy (P < 0.05).

CONCLUSION

Early chelating age will negatively influence the VEGF level, which increases angiogenesis, however, early starting transfusion age and regular blood transfusion will positively influence the VEGF level, which decreases angiogenesis in thalassemic patients.

Molecular mechanisms of allorecognition in a basal chordate

Allorecognition has been described in many metazoan phyla, from the sponges to the mammals. In vertebrates, allorecognition is a result of a MHC-based recognition event central to adaptive immunity. However, the origin of the adaptive immune system and the potential relationship to more primitive allorecognition systems is unclear. The colonial ascidian, Botryllus schlosseri, has been used as a model organism for the study of allorecognition for over a century, as it undergoes a natural transplantation reaction controlled by a single, highly polymorphic locus. Herein we will summarize our current understanding of the molecular mechanisms that underlie this innate allorecognition reaction.

Early lineage specification of long-lived germline precursors in the colonial ascidian Botryllus schlosseri

In many taxa, germline precursors segregate from somatic lineages during embryonic development and are irreversibly committed to gametogenesis. However, in animals that can propagate asexually, germline precursors can originate in adults. Botryllus schlosseri is a colonial ascidian that grows by asexual reproduction, and on a weekly basis regenerates all somatic and germline tissues. Embryonic development in solitary ascidians is the classic example of determinative specification, and we are interested in both the origins and the persistence of stem cells responsible for asexual development in colonial ascidians. In this study, we characterized vasa as a putative marker of germline precursors. We found that maternally deposited vasa mRNA segregates early in development to a posterior lineage of cells, suggesting that germline formation is determinative in colonial ascidians. In adults, vasa expression was observed in the gonads, as well as in a population of mobile cells scattered throughout the open circulatory system, consistent with previous transplantation/reconstitution results. vasa expression was dynamic during asexual development in both fertile and infertile adults, and was also enriched in a population of stem cells. Germline precursors in juveniles could contribute to gamete formation immediately upon transplantation into fertile adults, thus vasa expression is correlated with the potential for gamete formation, which suggests that it is a marker for embryonically specified, long-lived germline progenitors. Transient vasa knockdown did not have obvious effects on germline or somatic development in adult colonies, although it did result in a profound heterochrony, suggesting that vasa might play a homeostatic role in asexual development.

Sea squirts and immune tolerance

Transplantation specificity and protective immunity occur in both adaptive and innate branches of the vertebrate immune system. Understanding the mechanisms that underlie specificity and self-tolerance of immune function has major significance, from preventing a rejection reaction after transplantation to dissecting the causes of autoimmune disease. The core of vertebrate immunity is the ability to discriminate between highly polymorphic ligands, and this process is also found in allorecognition systems throughout the metazoa. Botryllus schlosseri is a tunicate, the modern-day descendents of the phylum that made the transition between invertebrates and vertebrates. In addition, B. schlosseri undergoes a natural transplantation reaction, which is controlled by a single, highly polymorphic locus called fuhc, reminiscent of major histocompatibility complex (MHC)-based allorecognition. The life-history characteristics of Botryllus make it an excellent model to dissect the functional and developmental mechanisms underlying allorecognition, and have the potential to reveal novel insights into issues from innate recognition strategies to the evolution of genetic polymorphism. In addition, we hypothesize that allorecognition in Botryllus must be based on conserved processes that are fundamental to all immune function: education and tolerance, or the ontogeny and maintenance of specificity.

Functional analysis of Pitx during asexual regeneration in a basal chordate

Embryogenesis in ascidians is the classic example of mosaic development, yet within this phyla a number of colonial species exist which as adults can reproduce entire bodies asexually. The colonial ascidian Botryllus schlosseri is an excellent model to study this process: on a weekly basis it regenerates all somatic and germline tissues, and while these processes have been characterized morphologically at high-resolution over the last 70 years, almost nothing is known regarding the genetic basis of asexual development and its relationship to embryogenesis. In this study, we functionally characterized the role of the paired-related homeobox transcription factor, Pitx, during this regenerative process. During ascidian embryogenesis Pitx seems to be multifunctional and involved in the formation of multiple tissues, including the stomodeum, pituitary gland, and determination of left-right asymmetry, similar to other deuterostomes. Previous spatial-temporal expression studies during asexual regeneration in Botryllus adults suggest the same roles in this developmental program. Here, we analyzed Pitx function using RNA interference at distinct stages of asexual development. Pitx phenotypes were described focusing on each developmental stage both in vivo, and via histological analysis, and were found to correspond to expression patterns; with the exception that normal asymmetries in the gut were not affected by knockdown. As mRNA destruction is not instantaneous, we found that by tailoring our short interfering double-stranded RNA delivery different developmental processes could be studied independently. This allows a reverse genetic approach to dissect asexual developmental pathways, even in cases involving multifunctional, ubiquitously expressed genes like Pitx.

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.