Wnt signaling in cnidarians

Extensive loss of Wnt genes in Tardigrada

Background

Wnt genes code for ligands that activate signaling pathways during development in Metazoa. Through the canonical Wnt (cWnt) signaling pathway, these genes regulate important processes in bilaterian development, such as establishing the anteroposterior axis and posterior growth. In Arthropoda, Wnt ligands also regulate segment polarity, and outgrowth and patterning of developing appendages. Arthropods are part of a lineage called Panarthropoda that includes Onychophora and Tardigrada. Previous studies revealed potential roles of Wnt genes in regulating posterior growth, segment polarity, and growth and patterning of legs in Onychophora. Unlike most other panarthropods, tardigrades lack posterior growth, but retain segmentation and appendages. Here, we investigated Wnt genes in tardigrades to gain insight into potential roles that these genes play during development of the highly compact and miniaturized tardigrade body plan.

Results

We analyzed published genomes for two representatives of Tardigrada, Hypsibius exemplaris and Ramazzottius varieornatus. We identified single orthologs of Wnt4, Wnt5, Wnt9, Wnt11, and WntA, as well as two Wnt16 paralogs in both tardigrade genomes. We only found a Wnt2 ortholog in H. exemplaris. We could not identify orthologs of Wnt1, Wnt6, Wnt7, Wnt8, or Wnt10. We identified most other components of cWnt signaling in both tardigrade genomes. However, we were unable to identify an ortholog of arrow/Lrp5/6, a gene that codes for a Frizzled co-receptor of Wnt ligands. Additionally, we found that some other animals that have lost several Wnt genes and are secondarily miniaturized, like tardigrades, are also missing an ortholog of arrow/Lrp5/6. We analyzed the embryonic expression patterns of Wnt genes in H. exemplaris during developmental stages that span the establishment of the AP axis through segmentation and leg development. We detected expression of all Wnt genes in H. exemplaris besides one of the Wnt16 paralogs. During embryo elongation, expression of several Wnt genes was restricted to the posterior pole or a region between the anterior and posterior poles. Wnt genes were expressed in distinct patterns during segmentation and development of legs in H. exemplaris, rather than in broadly overlapping patterns.

Conclusions

Our results indicate that Wnt signaling has been highly modified in Tardigrada. While most components of cWnt signaling are conserved in tardigrades, we conclude that tardigrades have lost Wnt1, Wnt6, Wnt7, Wnt8, and Wnt10, along with arrow/Lrp5/6. Our expression data may indicate a conserved role of Wnt genes in specifying posterior identities during establishment of the AP axis. However, the loss of several Wnt genes and the distinct expression patterns of Wnt genes during segmentation and leg development may indicate that combinatorial interactions among Wnt genes are less important during tardigrade development compared to many other animals. Based on our results, and comparisons to previous studies, we speculate that the loss of several Wnt genes in Tardigrada may be related to a reduced number of cells and simplified development that accompanied miniaturization and anatomical simplification in this lineage.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01954-y.

High regenerative capacity is a general feature within colonial dendrophylliid corals (Anthozoa, Scleractinia)

The regenerative capacity of cnidarians plays an essential role in the maintenance and restoration of coral reef ecosystems by allowing faster recovery from disturbances and more efficient small-scale dispersal. However, in the case of invasive species, this property may contribute to their dispersal and success in nonnative habitats. Given that four Indo-Pacific members of the coral genus Tubastraea have invaded the Atlantic, here we evaluated the ability of three of these species (Tubastraea coccinea, Tubastraea diaphana, and Tubastraea micranthus) to regenerate from fragments of undifferentiated coral tissue to fully functional polyps in response to differences in food supply and fragment size. For comparative purposes, another colonial dendrophylliid (Dendrophyllia sp.) was included in the analyses. All dendrophylliids displayed regenerative ability and high survival rates that were independent of whether or not food was supplied or fragment size. However, regeneration rates varied between species and were influenced by fragment size. Temporal expression of key genes of the regenerative process (Wnt and FGF) was profiled during whole-body regeneration of T. coccinea, suggesting a remarkable regenerative ability of T. coccinea that points to its potential use as a laboratory model for the investigation of regeneration in colonial calcified anthozoans.

Indomethacin reproducibly induces metamorphosis in Cassiopea xamachana scyphistomae

Cassiopea xamachana jellyfish are an attractive model system to study metamorphosis and/or cnidarian–dinoflagellate symbiosis due to the ease of cultivation of their planula larvae and scyphistomae through their asexual cycle, in which the latter can bud new larvae and continue the cycle without differentiation into ephyrae. Then, a subsequent induction of metamorphosis and full differentiation into ephyrae is believed to occur when the symbionts are acquired by the scyphistomae. Although strobilation induction and differentiation into ephyrae can be accomplished in various ways, a controlled, reproducible metamorphosis induction has not been reported. Such controlled metamorphosis induction is necessary for an ensured synchronicity and reproducibility of biological, biochemical, and molecular analyses. For this purpose, we tested if differentiation could be pharmacologically stimulated as in Aurelia aurita, by the metamorphic inducers thyroxine, KI, NaI, Lugol’s iodine, H₂O₂, indomethacin, or retinol. We found reproducibly induced strobilation by 50 μM indomethacin after six days of exposure, and 10–25 μM after 7 days. Strobilation under optimal conditions reached 80–100% with subsequent ephyrae release after exposure. Thyroxine yielded inconsistent results as it caused strobilation occasionally, while all other chemicals had no effect. Thus, indomethacin can be used as a convenient tool for assessment of biological phenomena through a controlled metamorphic process in C. xamachana scyphistomae.

A transcriptional time-course analysis of oral vs. aboral whole-body regeneration in the Sea anemone Nematostella vectensis

Background

The ability of regeneration is essential for the homeostasis of all animals as it allows the repair and renewal of tissues and body parts upon normal turnover or injury. The extent of this ability varies greatly in different animals with the sea anemone Nematostella vectensis, a basal cnidarian model animal, displaying remarkable whole-body regeneration competence.

Results

In order to study this process in Nematostella we performed an RNA-Seq screen wherein we analyzed and compared the transcriptional response to bisection in the wound-proximal body parts undergoing oral (head) or aboral (tail) regeneration at several time points up to the initial restoration of the basic body shape. The transcriptional profiles of regeneration responsive genes were analyzed so as to define the temporal pattern of differential gene expression associated with the tissue-specific oral and aboral regeneration. The identified genes were characterized according to their GO (gene ontology) assignations revealing groups that were enriched in the regeneration process with particular attention to their affiliation to the major developmental signaling pathways. While some of the genes and gene groups thus analyzed were previously known to be active in regeneration, we have also revealed novel and surprising candidate genes such as cilia-associated genes that likely participate in this important developmental program.

Conclusions

This work highlighted the main groups of genes which showed polarization upon regeneration, notably the proteinases, multiple transcription factors and the Wnt pathway genes that were highly represented, all displaying an intricate temporal balance between the two sides. In addition, the evolutionary comparison performed between regeneration in different animal model systems may reveal the basic mechanisms playing a role in this fascinating process.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3027-1) contains supplementary material, which is available to authorized users.

Frizzled-Induced Van Gogh Phosphorylation by CK1ε Promotes Asymmetric Localization of Core PCP Factors in Drosophila

Epithelial tissues are polarized along two axes. In addition to apical-basal polarity, they are often polarized within the plane of the epithelium, so-called Planar Cell Polarity (PCP). PCP depends upon Wnt/Frizzled (Fz) signaling factors, including Fz itself and Van Gogh (Vang/Vangl). We sought to understand how Vang interaction with other core PCP factors affects Vang function. We find that Fz induces Vang phosphorylation in a cell-autonomous manner. Vang phosphorylation occurs on conserved N-terminal serine/threonine residues, is mediated by CK1ε/Dco, and is critical for polarized membrane localization of Vang and other PCP proteins. This regulatory mechanism does not require Fz signaling through Dishevelled and thus represents a cell-autonomous upstream interaction between Fz and Vang. Furthermore, this signaling event appears to be related to Wnt5a-mediated Vangl2 phosphorylation during mouse limb patterning and may thus be a general mechanism underlying Wnt-regulated PCP establishment.

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.

Evidence for involvement of Wnt signalling in body polarities, cell proliferation, and the neuro-sensory system in an adult ctenophore

Signalling through the Wnt family of secreted proteins originated in a common metazoan ancestor and greatly influenced the evolution of animal body plans. In bilaterians, Wnt signalling plays multiple fundamental roles during embryonic development and in adult tissues, notably in axial patterning, neural development and stem cell regulation. Studies in various cnidarian species have particularly highlighted the evolutionarily conserved role of the Wnt/β-catenin pathway in specification and patterning of the primary embryonic axis. However in another key non-bilaterian phylum, Ctenophora, Wnts are not involved in early establishment of the body axis during embryogenesis. We analysed the expression in the adult of the ctenophore Pleurobrachia pileus of 11 orthologues of Wnt signalling genes including all ctenophore Wnt ligands and Fz receptors and several members of the intracellular β-catenin pathway machinery. All genes are strongly expressed around the mouth margin at the oral pole, evoking the Wnt oral centre of cnidarians. This observation is consistent with primary axis polarisation by the Wnts being a universal metazoan feature, secondarily lost in ctenophores during early development but retained in the adult. In addition, local expression of Wnt signalling genes was seen in various anatomical structures of the body including in the locomotory comb rows, where their complex deployment suggests control by the Wnts of local comb polarity. Other important contexts of Wnt involvement which probably evolved before the ctenophore/cnidarian/bilaterian split include proliferating stem cells and progenitors irrespective of cell types, and developing as well as differentiated neuro-sensory structures.

Ectopic activation of the canonical wnt signaling pathway affects ectodermal patterning along the primary axis during larval development in the anthozoan Nematostella vectensis

The primary axis of cnidarians runs from the oral pole to the apical tuft and defines the major body axis of both the planula larva and adult polyp. In the anthozoan cnidarian Nematostella vectensis, the primary oral-aboral (O-Ab) axis first develops during the early embryonic stage. Here, we present evidence that pharmaceutical activators of canonical wnt signaling affect molecular patterning along the primary axis of Nematostella. Although not overtly morphologically complex, molecular investigations in Nematostella reveal that the O-Ab axis is demarcated by the expression of differentially localized signaling molecules and transcription factors that may serve roles in establishing distinct ectodermal domains. We have further characterized the larval epithelium by determining the position of a nested set of molecular boundaries, utilizing several newly characterized as well as previously reported epithelial markers along the primary axis. We have assayed shifts in their position in control embryos and in embryos treated with the pharmacological agents alsterpaullone and azakenpaullone, Gsk3β inhibitors that act as canonical wnt agonists, and the Wnt antagonist iCRT14, following gastrulation. Agonist drug treatments result in an absence of aboral markers, a shift in the expression boundaries of oral markers toward the aboral pole, and changes in the position of differentially localized populations of neurons in a dose-dependent manner, while antagonist treatment had the opposite effect. These experiments are consistent with canonical wnt signaling playing a role in an orally localized wnt signaling center. These findings suggest that in Nematostella, wnt signaling mediates O-Ab ectodermal patterning across a surprisingly complex epithelium in planula stages following gastrulation in addition to previously described roles for the wnt signaling pathway in endomesoderm specification during gastrulation and overall animal-vegetal patterning at earlier stages of anthozoan development.

Wnt signaling in vertebrate axis specification

The Wnt pathway is a major embryonic signaling pathway that controls cell proliferation, cell fate, and body-axis determination in vertebrate embryos. Soon after egg fertilization, Wnt pathway components play a role in microtubule-dependent dorsoventral axis specification. Later in embryogenesis, another conserved function of the pathway is to specify the anteroposterior axis. The dual role of Wnt signaling in Xenopus and zebrafish embryos is regulated at different developmental stages by distinct sets of Wnt target genes. This review highlights recent progress in the discrimination of different signaling branches and the identification of specific pathway targets during vertebrate axial development.

Molecular signatures of the three stem cell lineages in hydra and the emergence of stem cell function at the base of multicellularity

How distinct stem cell populations originate and whether there is a clear stem cell "genetic signature" remain poorly understood. Understanding the evolution of stem cells requires molecular profiling of stem cells in an animal at a basal phylogenetic position. In this study, using transgenic Hydra polyps, we reveal for each of the three stem cell populations a specific signature set of transcriptions factors and of genes playing key roles in cell type-specific function and interlineage communication. Our data show that principal functions of stem cell genes, such as maintenance of stemness and control of stem cell self-renewal and differentiation, arose very early in metazoan evolution. They are corroborating the view that stem cell types shared common, multifunctional ancestors, which achieved complexity through a stepwise segregation of function in daughter cells.

The role of WNT10B in physiology and disease

Wnt10b is a member of the Wnt ligand gene family that encodes for secreted proteins, which activate the ancient and highly conserved Wnt signalling cascade. The Wnt pathway has been shown to be essential for embryonic development, tissue integrity, and stem cell activity, but if deregulated, also causes disease such as cancer. Although the 19 different Wnt ligands found in both human and mouse can activate several branches of the Wnt pathway, WNT10B specifically activates canonical Wnt/β-catenin signalling and thus triggers β-catenin/LEF/TCF-mediated transcriptional programs. In this review, we highlight the unique functions of WNT10B and mechanisms of how WNT10B acts in the immune system, mammary gland, adipose tissue, bone and skin. In these organs, WNT10B has been well established to be involved in signalling networks controlling stemness, pluripotency and cell fate decisions. Deregulation of these processes causes diseases such as breast cancer, obesity and osteoporosis. Compelling evidence suggests that WNT10B is a valuable candidate for the development of therapeutic regimens for human diseases.

Expression of secreted Wnt pathway components reveals unexpected complexity of the planarian amputation response

Regeneration is widespread throughout the animal kingdom, but our molecular understanding of this process in adult animals remains poorly understood. Wnt/β-catenin signaling plays crucial roles throughout animal life from early development to adulthood. In intact and regenerating planarians, the regulation of Wnt/β-catenin signaling functions to maintain and specify anterior/posterior (A/P) identity. Here, we explore the expression kinetics and RNAi phenotypes for secreted members of the Wnt signaling pathway in the planarian Schmidtea mediterranea. Smed-wnt and sFRP expression during regeneration is surprisingly dynamic and reveals fundamental aspects of planarian biology that have been previously unappreciated. We show that after amputation, a wounding response precedes rapid re-organization of the A/P axis. Furthermore, cells throughout the body plan can mount this response and reassess their new A/P location in the complete absence of stem cells. While initial stages of the amputation response are stem cell independent, tissue remodeling and the integration of a new A/P address with anatomy are stem cell dependent. We also show that WNT5 functions in a reciprocal manner with SLIT to pattern the planarian mediolateral axis, while WNT11-2 patterns the posterior midline. Moreover, we perform an extensive phylogenetic analysis on the Smed-wnt genes using a method that combines and integrates both sequence and structural alignments, enabling us to place all nine genes into Wnt subfamilies for the first time.

Emerging links between CDK cell cycle regulators and Wnt signaling

Wnt/beta-catenin signaling controls many aspects of cell behavior throughout development and in adults. One of its best-known and cancer-relevant functions is to stimulate cell proliferation. Recent work has implicated Wnt components in regulating mitotic events, suggesting that the cell cycle and Wnt signaling are directly linked. This concept has now been substantially strengthened with the finding that the mitotic CDK14/cyclin Y complex promotes Wnt signaling through phosphorylation of the LRP6 co-receptor, a key regulatory nexus in the Wnt/beta-catenin pathway. Thus, an unexpectedly tight collaboration between the mitotic cell cycle machinery and Wnt signaling is emerging, suggesting that this pathway might orchestrate mitotic processes.

Preparation techniques for transmission electron microscopy of Hydra

Hydra is a classical model organism in developmental and cell biology with a simple body plan reminiscent of a gastrula with one body axis and a limited number of cell types. This rather simple organism exhibits a regeneration capacity that is unique among all eumetazoans and is largely dependent on the stem cell properties of its epithelial stem cell population. Molecular work in the past few years has revealed an unexpected genetic complexity of these simple animals, making them an interesting model for studying the generation of animal form and regeneration. In addition, Hydra has an interstitial stem cell system with a unique population of nematocytes, neuronal cells that are characterized by an explosive exocytotic discharge. Here, we compare classical and modern transmission electron microscopy (TEM) fixation protocols including protocols for TEM immunocytochemistry (post-embedding immunogold labeling). We presume that TEM studies will become an important tool to analyze cell-cell interactions as well as cell matrix interrelationships in Hydra in the future.

Immortality and the base of multicellular life: Lessons from cnidarian stem cells

Cnidarians are phylogenetically basal members of the animal kingdom (>600 million years old). Together with plants they share some remarkable features that cannot be found in higher animals. Cnidarians and plants exhibit an almost unlimited regeneration capacity and immortality. Immortality can be ascribed to the asexual mode of reproduction that requires cells with an unlimited self-renewal capacity. We propose that the basic properties of animal stem cells are tightly linked to this archaic mode of reproduction. The cnidarian stem cells can give rise to a number of differentiated cell types including neuronal and germ cells. The genomes of Hydra and Nematostella, representatives of two major cnidarian classes indicate a surprising complexity of both genomes, which is in the range of vertebrates. Recent work indicates that highly conserved signalling pathways control Hydra stem cell differentiation. Furthermore, the availability of genomic resources and novel technologies provide approaches to analyse these cells in vivo. Studies of stem cells in cnidarians will therefore open important insights into the basic mechanisms of stem cell biology. Their critical phylogenetic position at the base of the metazoan branch in the tree of life makes them an important link in unravelling the common mechanisms of stem cell biology between animals and plants.

Emerging patterns in planarian regeneration

In the past decade, the planarian has become an increasingly tractable invertebrate model for the investigation of regeneration and stem cell biology. Application of a variety of techniques and development of genomic reagents in this system have enabled exploration of the molecular mechanisms by which pluripotent somatic stem cells called neoblasts replenish, repair, and regenerate planarian tissues and organs. Recent investigations have implicated evolutionarily conserved signaling pathways in the re-establishment of anterior-posterior (A-P), dorsal-ventral (D-V), and medial-lateral (M-L) polarity after injury. These studies have significantly advanced our understanding of early events during planarian regeneration and have raised new questions about the mechanisms of stem cell-based tissue repair and renewal.