Evolution of glial cells: a non-bilaterian perspective.

Nervous systems of bilaterian animals generally consist of two cell types: neurons and glial cells. Despite accumulating data about the many important functions glial cells serve in bilaterian nervous systems, the evolutionary origin of this abundant cell type remains unclear. Current hypotheses regarding glial evolution are mostly based on data from model bilaterians. Non-bilaterian animals have been largely overlooked in glial studies and have been subjected only to morphological analysis. Here, we provide a comprehensive overview of conservation of the bilateral gliogenic genetic repertoire of non-bilaterian phyla (Cnidaria, Placozoa, Ctenophora, and Porifera). We overview molecular and functional features of bilaterian glial cell types and discuss their possible evolutionary history. We then examine which glial features are present in non-bilaterians. Of these, cnidarians show the highest degree of gliogenic program conservation and may therefore be crucial to answer questions about glial evolution.

Regeneration Potential of Jellyfish: Cellular Mechanisms and Molecular Insights.

Medusozoans, the Cnidarian subphylum, have multiple life stages including sessile polyps and free-swimming medusae or jellyfish, which are typically bell-shaped gelatinous zooplanktons that exhibit diverse morphologies. Despite having a relatively complex body structure with well-developed muscles and nervous systems, the adult medusa stage maintains a high regenerative ability that enables organ regeneration as well as whole body reconstitution from the part of the body. This remarkable regeneration potential of jellyfish has long been acknowledged in different species; however, recent studies have begun dissecting the exact processes underpinning regeneration events. In this article, we introduce the current understanding of regeneration mechanisms in medusae, particularly focusing on cellular behaviors during regeneration such as wound healing, blastema formation by stem/progenitor cells or cell fate plasticity, and the organism-level patterning that restores radial symmetry. We also discuss putative molecular mechanisms involved in regeneration processes and introduce a variety of novel model jellyfish species in the effort to understand common principles and diverse mechanisms underlying the regeneration of complex organs and the entire body.

Cell-free scaffold from jellyfish Cassiopea andromeda (Cnidaria; Scyphozoa) for skin tissue engineering.

Disruption of the continuous cutaneous membrane in the integumentary system is considered a health problem of high cost for any nation. Several attempts have been made for developing skin substitutes in order to restore injured tissue including autologous implants and the use of scaffolds based on synthetic and natural materials. Current biomaterials used for skin tissue repair include several scaffold matrices types, synthetic or natural, absorbable, degradable or non-degradable polymers, porous or dense scaffolds, and cells capsulated in hydrogels or spheroids systems so forth. These materials have advantages and disadvantages and its use will depend on the desired application. Recently, marine organisms such as jellyfish have attracted renewed interest, because both its composition and structure resemble the architecture of human dermic tissue. In this context, the present study aims to generate scaffolds from Cassiopea andromeda (C. andromeda), with application in skin tissue engineering, using a decellularization process. The obtained scaffold was studied by infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), differential scanning calorimetry analysis (DSC), and scanning electron microscopy (SEM). Crystal violet staining and DNA quantification assessed decellularization effectiveness while the biocompatibility of scaffold was determined with human dermic fibroblasts. Results indicated that the decellularization process reduce native cell population leading to 70% reduction in DNA content. In addition, SEM showed that the macro and microstructure of the collagen I-based scaffold were preserved allowing good adhesion and proliferation of human dermic fibroblasts. The C. andromeda scaffold mimics human skin and therefore represents great potential for skin tissue engineering.

A cell-based boundary model of gastrulation by unipolar ingression in the hydrozoan cnidarian Clytia hemisphaerica.

In Cnidaria, modes of gastrulation to produce the two body layers vary greatly between species. In the hydrozoan species Clytia hemisphaerica gastrulation involves unipolar ingression of presumptive endoderm cells from an oral domain of the blastula, followed by migration of these cells to fill the blastocoel with concomitant narrowing of the gastrula and elongation along the oral-aboral axis. We developed a 2D computational boundary model capable of simulating the morphogenetic changes during embryonic development from early blastula stage to the end of gastrulation. Cells are modeled as polygons with elastic membranes and cytoplasm, colliding and adhering to other cells, and capable of forming filopodia. With this model we could simulate compaction of the embryo preceding gastrulation, bottle cell formation, ingression, and intercalation between cells of the ingressing presumptive endoderm. We show that embryo elongation is dependent on the number of endodermal cells, low endodermal cell-cell adhesion, and planar cell polarity (PCP). When the strength of PCP is reduced in our model, resultant embryo morphologies closely resemble those reported previously following morpholino-mediated knockdown of the core PCP proteins Strabismus and Frizzled. Based on our results, we postulate that cellular processes of apical constriction, compaction, ingression, and then reduced cell-cell adhesion and mediolateral intercalation in the presumptive endoderm, are required and when combined, sufficient for Clytia gastrulation.

Epithelial folding in the morphogenesis of the colonial marine hydrozoan, Dynamena pumila.

Epithelial folding (EF) is a fundamental morphogenetic process that can be observed in the development of many organisms ranging from metazoans to green algae. Being early branching metazoans, cnidarians represent the best models to study evolutionarily conserved morphogenetic processes, including EF. Hydrozoa is the most evolutionary advanced group of the phylum Cnidaria. All colonial hydrozoans grow continuously, changing the shape of their colonies and spreading over the substrate with the help of elongating stolons. Owing to high diversity of colony architecture, they are ideal objects for comparative and evolutionary morphology. In the hydrozoan Dynamena pumila, the growth of the colony proceeds via a variety of morphogenetic processes. Our work is focused on the formation of the anchoring disc of the stolon, which is accompanied by inward-folding morphogenesis of the ectodermal layer. Successive stages of anchoring disc development were described with light, confocal transmission electron microscopy. We have shown that EF in Dynamena is associated with accumulation of F-actin in the constricting apical domains of forming bottle cells located at the bottom of the emerging fold. In addition, the nuclei of these cells are displaced to the basal domains. Taken together, these features may indicate that EF in Dynamena proceeds as an active invagination, although this process has never been described in the development of hydrozoans. Apparently, development of the anchoring disc can be viewed as a reliable and versatile model system for studying the cell-shape-change-driven epithelial sheet morphogenesis, which can be easily observed and analysed.

[The gastrulation in Cnidaria: A key to understanding phylogeny or the chaos of secondary modifications?].

The data revealed by comparative embryology of the basal (diploblastic) metazoans is traditionally considered a valuable potential source of information on the origin and early evolution of the animal kingdom and its major clades. Special attention is paid to the fundamental morphogenetic process of gastrulation during which the cells of the early embryo differentiate into the germ layers and the primary body plan is formed. Comparative analysis of gastrulation in different cnidarian taxa reveals high level of intergroup, intragroup, and individual variation. With few exceptions, there is no robust correlation between the type of gastrulation and the taxon. Current data do not support the idea that morphogenetic processes underlying cnidarian gastrulation can be divided into several distinct types. Rather, there is a continuum of equifinal ontogenetic trajectories. In cnidarians, the mode of gastrulation apparently depends less on the macroevolutionary history of the species than on various evolutionary plastic features, such as the oocyte size, the amount of yolk, the number of cells at the blastula (or morula) stage, the presence of phototrophic symbionts, or the ecology of the larva. Thus, in cnidarians, morphogenetic basis of gastrulation contains only a very weak phylogenetic signal and can have only limited application in phylogenetic reconstructions. On the other hand, comparative studies of the ontogeny of the basal metazoans shed light on the general rules of the evolution of morphogenetic processes that is crucial for understanding the early history of the animal kingdom.

Nervous system immunohistochemistry of the parasitic cnidarian Polypodium hydriforme at its free-living stage.

Polypodium hydriforme, the only species in Polypodiozoa, which is currently considered a class of Cnidaria, and likely a sister group to Medusozoa (together with Myxozoa), is a cnidarian adapted to intracellular parasitism inside sturgeon oocytes. Free-living P. hydriforme lives on river bottoms; it walks on supporting tentacles and uses sensory tentacles to capture food and bring it to the mouth. The nervous system of free-living P. hydriforme was studied by confocal microscopy and immunohistochemistry using antibodies to FMRF-amide and α-tubulin combined with phalloidin-staining of F-actin fibres. A sensory FMRF-amide immunoreactive (IR) nerve net and an α-tubulin IR nerve net have been identified. The FMRF-amide IR nerve net underlies the epidermis along the tentacles and around the mouth; it consists of neurites emanating from epidermal sensory cells and basiepidermal ganglion cells, and it connects with cnidocytes. A deeper-lying α-tubulin IR nerve net occurs only in tentacles and looks like chains of different-sized beads crossing the mesoglea and entwining muscles. Anti-α-tubulin staining also reveals microtubules in muscle cells following the longitudinal muscle fibres or the thin circular F-actin fibres of the tentacles. Cnidocytes in the tentacles are embedded in a regular hexagonal non-neural network formed by the tubulin IR cytoskeleton of epidermal cells. Cnidocils of the cnidocytes around the mouth and in walking tentacles are identical, but those in sensory tentacles differ in length and width. The possible homology of the tubulin IR nerve net with motor nerve nets of cnidarians is discussed. The absence of a classic nerve ring around the mouth and the lack of specialised sense organs are considered to be plesiomorphic characters for Cnidaria.

Meeting report on "Animal Evolution: New Perspectives From Early Emerging Metazoans", Tutzing, September 14-17, 2015.

This recent meeting covered non-bilaterian (e.g., cnidarians, ctenophores, and sponges) animals broadly, but with emphasis in four areas: 1) New genomic resources and tools for functional studies, 2) advances in developmental and regeneration studies, 3) the evolution and function of nervous systems, 4) symbiosis and the holobiome.

Rapid, Precise, and Accurate Counts of Symbiodinium Cells Using the Guava Flow Cytometer, and a Comparison to Other Methods.

In studies of both the establishment and breakdown of cnidarian-dinoflagellate symbiosis, it is often necessary to determine the number of Symbiodinium cells relative to the quantity of host tissue. Ideally, the methods used should be rapid, precise, and accurate. In this study, we systematically evaluated methods for sample preparation and storage and the counting of algal cells using the hemocytometer, a custom image-analysis program for automated counting of the fluorescent algal cells, the Coulter Counter, or the Millipore Guava flow-cytometer. We found that although other methods may have value in particular applications, for most purposes, the Guava flow cytometer provided by far the best combination of precision, accuracy, and efficient use of investigator time (due to the instrument's automated sample handling), while also allowing counts of algal numbers over a wide range and in small volumes of tissue homogenate. We also found that either of two assays of total homogenate protein provided a precise and seemingly accurate basis for normalization of algal counts to the total amount of holobiont tissue.

Distinct mechanisms underlie oral vs aboral regeneration in the cnidarian Hydractinia echinata.

Cnidarians possess remarkable powers of regeneration, but the cellular and molecular mechanisms underlying this capability are unclear. Studying the hydrozoan Hydractinia echinata we show that a burst of stem cell proliferation occurs following decapitation, forming a blastema at the oral pole within 24 hr. This process is necessary for head regeneration. Knocking down Piwi1, Vasa, Pl10 or Ncol1 expressed by blastema cells inhibited regeneration but not blastema formation. EdU pulse-chase experiments and in vivo tracking of individual transgenic Piwi1(+) stem cells showed that the cellular source for blastema formation is migration of stem cells from a remote area. Surprisingly, no blastema developed at the aboral pole after stolon removal. Instead, polyps transformed into stolons and then budded polyps. Hence, distinct mechanisms act to regenerate different body parts in Hydractinia. This model, where stem cell behavior can be monitored in vivo at single cell resolution, offers new insights for regenerative biology.

Old cell, new trick? Cnidocytes as a model for the evolution of novelty.

Understanding how new cell types arise is critical for understanding the evolution of organismal complexity. Questions of this nature, however, can be difficult to answer due to the challenge associated with defining the identity of a truly novel cell. Cnidarians (anemones, jellies, and their allies) provide a unique opportunity to investigate the molecular regulation and development of cell-novelty because they possess a cell that is unique to the cnidarian lineage and that also has a very well-characterized phenotype: the cnidocyte (stinging cell). Because cnidocytes are thought to differentiate from the cell lineage that also gives rise to neurons, cnidocytes can be expected to express many of the same genes expressed in their neural "sister" cells. Conversely, only cnidocytes posses a cnidocyst (the explosive organelle that gives cnidocytes their sting); therefore, those genes or gene-regulatory relationships required for the development of the cnidocyst can be expected to be expressed uniquely (or in unique combination) in cnidocytes. This system provides an important opportunity to: (1) construct the gene-regulatory network (GRN) underlying the differentiation of cnidocytes, (2) assess the relative contributions of both conserved and derived genes in the cnidocyte GRN, and (3) test hypotheses about the role of novel regulatory relationships in the generation of novel cell types. In this review, we summarize common challenges to studying the evolution of novelty, introduce the utility of cnidocyte differentiation in the model cnidarian, Nematostella vectensis, as a means of overcoming these challenges, and describe an experimental approach that leverages comparative tissue-specific transcriptomics to generate hypotheses about the GRNs underlying the acquisition of the cnidocyte identity.

Initiating a regenerative response; cellular and molecular features of wound healing in the cnidarian Nematostella vectensis.

BACKGROUND: Wound healing is the first stage of a series of cellular events that are necessary to initiate a regenerative response. Defective wound healing can block regeneration even in animals with a high regenerative capacity. Understanding how signals generated during wound healing promote regeneration of lost structures is highly important, considering that virtually all animals have the ability to heal but many lack the ability to regenerate missing structures. Cnidarians are the phylogenetic sister taxa to bilaterians and are highly regenerative animals. To gain a greater understanding of how early animals generate a regenerative response, we examined the cellular and molecular components involved during wound healing in the anthozoan cnidarian Nematostella vectensis. RESULTS: Pharmacological inhibition of extracellular signal-regulated kinases (ERK) signaling blocks regeneration and wound healing in Nematostella. We characterized early and late wound healing events through genome-wide microarray analysis, quantitative PCR, and in situ hybridization to identify potential wound healing targets. We identified a number of genes directly related to the wound healing response in other animals (metalloproteinases, growth factors, transcription factors) and suggest that glycoproteins (mucins and uromodulin) play a key role in early wound healing events. This study also identified a novel cnidarian-specific gene, for a thiamine biosynthesis enzyme (vitamin B synthesis), that may have been incorporated into the genome by lateral gene transfer from bacteria and now functions during wound healing. Lastly, we suggest that ERK signaling is a shared element of the early wound response for animals with a high regenerative capacity. CONCLUSIONS: This research describes the temporal events involved during Nematostella wound healing, and provides a foundation for comparative analysis with other regenerative and non-regenerative species. We have shown that the same genes that heal puncture wounds are also activated after oral-aboral bisection, indicating a clear link with the initiation of regenerative healing. This study demonstrates the strength of using a forward approach (microarray) to characterize a developmental phenomenon (wound healing) at a phylogenetically important crossroad of animal evolution (cnidarian-bilaterian ancestor). Accumulation of data on the early wound healing events across numerous systems may provide clues as to why some animals have limited regenerative abilities.

Adhesion networks of cnidarians: a postgenomic view.

Cell-extracellular matrix (ECM) and cell-cell adhesion systems are fundamental to the multicellularity of metazoans. Members of phylum Cnidaria were classified historically by their radial symmetry as an outgroup to bilaterian animals. Experimental study of Hydra and jellyfish has fascinated zoologists for many years. Laboratory studies, based on dissection, biochemical isolations, or perturbations of the living organism, have identified the ECM layer of cnidarians (mesoglea) and its components as important determinants of stem cell properties, cell migration and differentiation, tissue morphogenesis, repair, and regeneration. Studies of the ultrastructure and functions of intercellular gap and septate junctions identified parallel roles for these structures in intercellular communication and morphogenesis. More recently, the sequenced genomes of sea anemone Nematostella vectensis, Hydra magnipapillata, and coral Acropora digitifera have opened up a new frame of reference for analyzing the cell-ECM and cell-cell adhesion molecules of cnidarians and examining their conservation with bilaterians. This chapter integrates a review of literature on the structure and functions of cell-ECM and cell-cell adhesion systems in cnidarians with current analyses of genome-encoded repertoires of adhesion molecules. The postgenomic perspective provides a fresh view on fundamental similarities between cnidarian and bilaterian animals and is impelling wider adoption of species from phylum Cnidaria as model organisms.

The genomic and cellular foundations of animal origins.

The first animals arose more than six hundred million years ago, yet they left little impression in the fossil record. Nonetheless, the cell biology and genome composition of the first animal, the Urmetazoan, can be reconstructed through the study of phylogenetically relevant living organisms. Comparisons among animals and their unicellular and colonial relatives reveal that the Urmetazoan likely possessed a layer of epithelium-like collar cells, preyed on bacteria, reproduced by sperm and egg, and developed through cell division, cell differentiation, and invagination. Although many genes involved in development, body patterning, immunity, and cell-type specification evolved in the animal stem lineage or after animal origins, several gene families critical for cell adhesion, signaling, and gene regulation predate the origin of animals. The ancestral functions of these and other genes may eventually be revealed through studies of gene and genome function in early-branching animals and their closest non-animal relatives.

Stress and death of cnidarian host cells play a role in cnidarian bleaching.

Coral bleaching occurs when there is a breakdown of the symbiosis between cnidarian hosts and resident Symbiodinium spp. Multiple mechanisms for the bleaching process have been identified, including apoptosis and autophagy, and most previous work has focused on the Symbiodinium cell as the initiator of the bleaching cascade. In this work we show that it is possible for host cells to initiate apoptosis that can contribute to death of the Symbiodinium cell. First we found that colchicine, which results in apoptosis in other animals, causes cell death in the model anemone Aiptasia sp. but not in cultured Symbiodinium CCMP-830 cells or in cells freshly isolated from host Aiptasia (at least within the time frame of our study). In contrast, when symbiotic Aiptasia were incubated in colchicine, cell death in the resident Symbiodinium cells was observed, suggesting a host effect on symbiont mortality. Using live-cell confocal imaging of macerated symbiotic host cell isolates, we identified a pattern where the initiation of host cell death was followed by mortality of the resident Symbiodinium cells. This same pattern was observed in symbiotic host cells that were subjected to temperature stress. This research suggests that mortality of symbionts during temperature-induced bleaching can be initiated in part by host cell apoptosis.

Heavy metals affect regulatory volume decrease (RVD) in nematocytes isolated from the jellyfish Pelagia noctiluca.

The environmental contamination caused by heavy metals raises the question of their effect on biological systems. Among bio-indicators useful to monitor the toxicological effects of these chemicals, Cnidarians offer a unique model. Cnidarians possess highly specialized stinging cells, termed nematocytes, which respond to hyposmotic solution with well established homeostatic parameters as an acute osmotic phase (OP), leading to cell swelling, and then a slower regulatory volume decrease (RVD) phase, causing cell shrinkage. Here we report the effect of 65% artificial sea water (ASW) containing heavy metals, such as Cd, La, Co, Cu and Zn (concentrations comprised between 100 and 0.1 µM) on both OP and RVD in nematocytes isolated from the jellyfish Pelagia noctiluca by 605 mM NaSCN plus 0.01 mM Ca(2+). The exposure of the cells to Co and La inhibited RVD but not OP. However, Cu, Cd and Zn prevented the OP in a dose-dependent manner and, hence, also the detection of RVD. These results suggest that, in isolated nematocytes, heavy metal pollutants impair RVD either directly or indirectly through interference with the OP, thus negating RVD. Although further studies need to clarify the exact mechanisms whereby heavy metals exert their toxicity, it is evident that nematocytes of Cnidarians could serve as a model for ecotoxicological investigations.

Allorecognition triggers autophagy and subsequent necrosis in the cnidarian Hydractinia symbiolongicarpus.

Transitory fusion is an allorecognition phenotype displayed by the colonial hydroid Hydractinia symbiolongicarpus when interacting colonies share some, but not all, loci within the allorecognition gene complex (ARC). The phenotype is characterized by an initial fusion followed by subsequent cell death resulting in separation of the two incompatible colonies. We here characterize this cell death process using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and continuous in vivo digital microscopy. These techniques reveal widespread autophagy and subsequent necrosis in both colony and grafted polyp assays. Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assays and ultrastructural observations revealed no evidence of apoptosis. Pharmacological inhibition of autophagy using 3-methyladenine (3-MA) completely suppressed transitory fusion in vivo in colony assays. Rapamycin did not have a significant effect in the same assays. These results establish the hydroid allorecognition system as a novel model for the study of cell death.

Incorporated nematocysts in Aeolidiella stephanieae (Gastropoda, Opisthobranchia, Aeolidoidea) mature by acidification shown by the pH sensitive fluorescing alkaloid Ageladine A.

The sequestration of nematocysts (a special group of cnidocysts) from cnidarian prey with subsequent use in defence is described for few metazoan phyla. Members of the taxon Aeolidoidea (Nudibranchia, Gastropoda) are well-known for this. Questions regarding the reasons some nematocysts do not discharge when the gastropod feeds and how these same nematocysts can be transported along the digestive tract into specialized morphological structures called cnidosacs, remain unanswered. Within the cnidosac, nematocysts are incorporated in cells and finally be used for defence against predators. The most plausible explanation for this phenomenon suggests there are immature and therefore non-functional nematocysts in the food. A recent study by Berking and Herrmann (2005) on cnidarians suggested that the nematocysts mature by acidification via proton transfer into the nematocyst capsule. According to this hypothesis only immature nematocysts are transported into the cnidosac where they are then made functional through an accumulation of protons. In this study we present a fluorescence staining method that tests the hypothesis by Berking and Herrmann (2005) and detects changes in the pH values of incorporated nematocysts, interpreted as changes in maturation stages. This marker, the fluorescent dye Ageladine A, stains nematocyst capsules according to their pH values. With Ageladine A we were able to show that kleptocnides indeed change their pH value after incorporation into the aeolidoidean cnidosac.

Thermal stress promotes host mitochondrial degradation in symbiotic cnidarians: are the batteries of the reef going to run out?

The symbiotic relationship between cnidarians and their dinoflagellate symbionts, Symbiodinium spp, which underpins the formation of tropical coral reefs, can be destabilized by rapid changes to environmental conditions. Although some studies have concluded that a breakdown in the symbiosis begins with increased reactive oxygen species (ROS) generation within the symbiont due to a decoupling of photosynthesis, others have reported the release of viable symbionts via a variety of host cell derived mechanisms. We explored an alternative model focused upon changes in host cnidarian mitochondrial integrity in response to thermal stress. Mitochondria are often likened to being batteries of the cell, providing energy in the form of ATP, and controlling cellular pathway activation and ROS generation. The overall morphology of host mitochondria was compared to that of associated symbionts under an experimental thermal stress using confocal and electron microscopy. The results demonstrate that hyperthermic stress induces the degradation of cnidarian host mitochondria that is independent of symbiont cellular deterioration. The potential sites of host mitochondrial disruption were also assessed by measuring changes in the expression of genes associated with electron transport and ATP synthesis using quantitative RT-PCR. The primary site of degradation appeared to be downstream of complex III of the electron transport chain with a significant reduction in host cytochrome c and ATP synthase expression. The consequences of reduced expression could limit the capacity of the host to mitigate ROS generation and maintain both organelle integrity and cellular energy supplies. The disruption of host mitochondria, cellular homeostasis, and subsequent cell death irrespective of symbiont integrity highlights the importance of the host response to thermal stress and in symbiosis dysfunction that has substantial implications for understanding how coral reefs will survive in the face of climate change.

Stem cells: a view from the roots.

In both plants and animals, regeneration requires the activation of stem cells. This is possibly related to the origin and requirements of multicellularity. Although long diverged from a common ancestry, plant and animal models such as Arabidopsis, Drosophila and mouse share considerable similarities in stem cell regulation. This includes stem cell niche organisation, epigenetic modification of DNA and histones, and the role of small RNA machinery in differentiation and pluripotency states. Dysregulation of any of these can lead to premature ageing, patterning and specification defects, as well as cancers. Moreover, emerging basal animal and plant systems are beginning to provide important clues concerning the diversity and evolutionary history of stem cell regulatory mechanisms in eukaryotes. This review provides a comparative framework, highlighting both the commonalities and differences among groups, which should promote the intelligent design of artificial stem cell systems, and thereby fuel the field of biomaterials science.