Origin of metazoan stem cell system in sponges: first approach to establish the model (Suberites domuncula)

Phylogenetic evidence for the modular evolution of metazoan signalling pathways

Communication among cells was paramount to the evolutionary increase in cell type diversity and, ultimately, the origin of large body size. Across the diversity of Metazoa, there are only few conserved cell signalling pathways known to orchestrate the complex cell and tissue interactions regulating development; thus, modification to these few pathways has been responsible for generating diversity during the evolution of animals. Here, we summarize evidence for the origin and putative function of the intracellular, membrane-bound and secreted components of seven metazoan cell signalling pathways with a special focus on early branching metazoans (ctenophores, poriferans, placozoans and cnidarians) and basal unikonts (amoebozoans, fungi, filastereans and choanoflagellates). We highlight the modular incorporation of intra- and extracellular components in each signalling pathway and suggest that increases in the complexity of the extracellular matrix may have further promoted the modulation of cell signalling during metazoan evolution. Most importantly, this updated view of metazoan signalling pathways highlights the need for explicit study of canonical signalling pathway components in taxa that do not operate a complete signalling pathway. Studies like these are critical for developing a deeper understanding of the evolution of cell signalling.This article is part of the themed issue 'Evo-devo in the genomics era, and the origins of morphological diversity'.

Agonists and Antagonists of TGF-β Family Ligands

The discovery of the transforming growth factor β (TGF-β) family ligands and the realization that their bioactivities need to be tightly controlled temporally and spatially led to intensive research that has identified a multitude of extracellular modulators of TGF-β family ligands, uncovered their functions in developmental and pathophysiological processes, defined the mechanisms of their activities, and explored potential modulator-based therapeutic applications in treating human diseases. These studies revealed a diverse repertoire of extracellular and membrane-associated molecules that are capable of modulating TGF-β family signals via control of ligand availability, processing, ligand-receptor interaction, and receptor activation. These molecules include not only soluble ligand-binding proteins that were conventionally considered as agonists and antagonists of TGF-β family of growth factors, but also extracellular matrix (ECM) proteins and proteoglycans that can serve as "sink" and control storage and release of both the TGF-β family ligands and their regulators. This extensive network of soluble and ECM modulators helps to ensure dynamic and cell-specific control of TGF-β family signals. This article reviews our knowledge of extracellular modulation of TGF-β growth factors by diverse proteins and their molecular mechanisms to regulate TGF-β family signaling.

Evo-devo: Hydra raises its Noggin

Noggin, along with other secreted bone morphogenetic protein (BMP) inhibitors, plays a crucial role in neural induction and neural tube patterning as well as in somitogenesis, cardiac morphogenesis and formation of the skeleton in vertebrates. The BMP signalling pathway is one of the seven fundamental pathways that drive embryonic development and pattern formation in animals. Understanding its evolutionary origin and role in pattern formation is, therefore, important to evolutionary developmental biology (evo-devo). We have studied the evolutionary origin of BMP-Noggin antagonism in hydra, which is a powerful diploblastic model to study evolution of pattern-forming mechanisms because of the unusual cellular dynamics during its pattern formation and its remarkable ability to regenerate. We cloned and characterized the noggin gene from hydra and found it to exhibit considerable similarity with its orthologues at the amino acid level. Microinjection of hydra Noggin mRNA led to duplication of the dorsoventral axis in Xenopus embryos, demonstrating its functional conservation across the taxa. Our data, along with those of others, indicate that the evolutionarily conserved antagonism between BMP and its inhibitors predates bilateral divergence. This article reviews the various roles of Noggin in different organisms and some of our recent work on hydra Noggin in the context of evolution of developmental signalling pathways.

Cellular localization of debromohymenialdisine and hymenialdisine in the marine sponge Axinella sp. using a newly developed cell purification protocol

Sponges (Porifera), as the best known source of bioactive marine natural products in metazoans, play a significant role in marine drug discovery and development. As sessile filter-feeding animals, a considerable portion of the sponge biomass can be made of endosymbiotic and associated microorganisms. Understanding the cellular origin of targeted bioactive compounds from sponges is therefore important not only for providing chemotaxonomic information but also for defining the bioactive production strategy in terms of sponge aquaculture, cell culture, or fermentation of associated bacteria. The two alkaloids debromohymenialdisine (DBH) and hymenialdisine (HD), which are cyclin-dependent kinase inhibitors with pharmacological activities for treating osteoarthritis and Alzheimer's disease, have been isolated from the sponge Axinella sp. In this study, the cellular localization of these two alkaloids was determined through the quantification of these alkaloids in different cell fractions by high-performance liquid chromatography (HPLC). First, using a differential centrifugation method, the dissociated cells were separated into different groups according to their sizes. The two bioactive alkaloids were mainly found in sponge cells obtained from low-speed centrifugation. Further cell purifications were accomplished by a newly developed multi-step protocol. Four enriched cell fractions (C1, C2, C3, and C4) were obtained and subjected to light and transmission electron microscopy, cytochemical staining, and HPLC quantification. Compared to the low concentrations in other cell fractions, DBH and HD accounted for 10.9% and 6.1%, respectively, of dry weight in the C1 fraction. Using the morphological characteristics and cytochemical staining results, cells in the C1 fraction were speculated to be spherulous cells. This result shows that DBH and HD in Axinella sp. are located in sponge cells and mostly stored in spherulous cells.

Inflammation meets cancer, with NF-κB as the matchmaker

Inflammation is a fundamental protective response that sometimes goes awry and becomes a major cofactor in the pathogenesis of many chronic human diseases, including cancer. Here we review the evolutionary relationship and opposing functions of the transcription factor NF-κB in inflammation and cancer. Although it seems to fulfill a distinctly tumor-promoting role in many types of cancer, NF-κB has a confounding role in certain tumors. Understanding the activity and function of NF-κB in the context of tumorigenesis is critical for its successful taming, an important challenge for modern cancer biology.

The stem cell system in demosponges: insights into the origin of somatic stem cells

The stem cell system is one of the unique systems that have evolved only in multicellular organisms. Major questions about this system include what type(s) of stem cells are involved (pluri-, multi- or uni-potent stem cells), and how the self-renewal and differentiation of stem cells are regulated. To understand the origin of the stem cell system in metazoans and to get insights into the ancestral stem cell itself, it is important to discover the molecular and cellular mechanisms of the stem cell system in sponges (Porifera), the evolutionarily oldest extant metazoans. Histological studies here provided a body of evidence that archeocytes are the stem cells in sponges, and recent molecular studies of sponges, especially the finding of the expression of Piwi homologues in archeocytes and choanocytes in a freshwater sponge, Ephydatia fluviatilis, have provided critical insights into the stem cell system in demosponges. Here I introduce archeocytes and discuss (i) modes of archeocyte differentiation, (ii) our current model of the stem cell system in sponges composed of both archeocytes and choanocytes based on our molecular analysis and previous microscopic studies suggesting the maintenance of pluripotency in choanocytes, (iii) the inference that the Piwi and piRNA function in maintaining stem cells (which also give rise to gametes) may have already been achieved in the ancestral metazoan, and (iv) possible hypotheses about how the migrating stem cells arose in the urmetazoan (protometazoan) and about the evolutionary origin of germline cells in the urbilaterian (protobilaterian).

Expression pattern of the expanded noggin gene family in the planarian Schmidtea mediterranea

Noggin genes are mainly known as inhibitors of the Bone Morphogenetic Protein (BMP) signalling pathway. Noggin genes play an important role in various developmental processes such as axis formation and neural differentiation. In vertebrates, inhibition of the BMP pathway is usually carried out together with other inhibitory molecules: chordin and follistatin. Recently, it has been shown in planarians that the BMP pathway has a conserved function in the maintenance and re-establishment of the dorsoventral axis during homeostasis and regeneration. In an attempt to further characterize the BMP pathway in this model we have undertaken an in silico search of noggin genes in the genome of Schmidtea mediterranea. In contrast to other systems in which between one and four noggin genes have been reported, ten genes containing a noggin domain are present in S. mediterranea. These genes have been classified into two groups: noggin genes (two genes) and noggin-like genes (eight genes). Noggin-like genes are characterized by the presence of an insertion of 50-60 amino acids in the middle of the noggin domain. Here, we report the characterization of this expanded family of noggin genes in planarians as well as their expression patterns in both intact and regenerating animals. In situ hybridizations show that planarian noggin genes are expressed in a variety of cell types located in different regions of the planarian body.

The evolutionary origin of the Runx/CBFbeta transcription factors--studies of the most basal metazoans

Background

Members of the Runx family of transcriptional regulators, which bind DNA as heterodimers with CBFβ, are known to play critical roles in embryonic development in many triploblastic animals such as mammals and insects. They are known to regulate basic developmental processes such as cell fate determination and cellular potency in multiple stem-cell types, including the sensory nerve cell progenitors of ganglia in mammals.

Results

In this study, we detect and characterize the hitherto unexplored Runx/CBFβ genes of cnidarians and sponges, two basal animal lineages that are well known for their extensive regenerative capacity. Comparative structural modeling indicates that the Runx-CBFβ-DNA complex from most cnidarians and sponges is highly similar to that found in humans, with changes in the residues involved in Runx-CBFβ dimerization in either of the proteins mirrored by compensatory changes in the binding partner. In situ hybridization studies reveal that Nematostella Runx and CBFβ are expressed predominantly in small isolated foci at the base of the ectoderm of the tentacles in adult animals, possibly representing neurons or their progenitors.

Conclusion

These results reveal that Runx and CBFβ likely functioned together to regulate transcription in the common ancestor of all metazoans, and the structure of the Runx-CBFβ-DNA complex has remained extremely conserved since the human-sponge divergence. The expression data suggest a hypothesis that these genes may have played a role in nerve cell differentiation or maintenance in the common ancestor of cnidarians and bilaterians.

Cell culture from sponges: pluripotency and immortality

Sponges are a source of compounds with potential pharmaceutical applications. In this article, methods of sponge cell culture for production of these bioactive compounds are reviewed, and new approaches for overcoming the problem of metabolite supply are examined. The use of embryos is proposed as a new source of sponge material for cell culture. Stem cells are present in high amounts in embryos and are more versatile and resistant to infections than adult cells. Additionally, genetic engineering and cellular research on apoptotic mechanisms are promising new fields that might help to improve cell survival in sponge-cell lines. We propose that one topic for future research should be how to reduce apoptosis, which appears to be very high in sponge cell cultures.

Blood Cells and Blood Cell Development in the Animal Kingdom

Recent findings strongly suggest that the molecular pathways involved in the development and function of blood cells are highly conserved among vertebrates and various invertebrate phyla. This has led to a renewed interest regarding homologies between blood cell types and their developmental origin among different animals. One way to address these areas of inquiry is to shed more light on the biology of blood cells in extant invertebrate taxa that have branched off the bilaterian tree in between insects and vertebrates. This review attempts, in a broadly comparative manner, to update the existing literature that deals with early blood cell development. I begin by providing a brief survey of the different types of blood cell lineages among metazoa. There is now good reason to believe that, in vertebrates and invertebrates alike, blood cell lineages diverge from a common type of progenitor cell, the hemocytoblast. I give a synopsis of the origin and determination of the hematocytoblast, beginning with a look at the hematopoietic organs that house hemocytoblasts in adult animals, followed by a more detailed overview of the embryonic development of the hematopoietic organ. Finally, I compare the process of blood lineage diversification in vertebrates and Drosophila.

The stem cell concept in sponges (Porifera): Metazoan traits

Sponges are considered the oldest living animal group and provide important insights into the earliest evolutionary processes in the Metazoa. This paper reviews the evidence that sponge stem cells have essential roles in cellular specialization, embryogenesis and Bauplan formation. Data indicate that sponge archaeocytes not only represent germ cells but also totipotent stem cells. Marker genes have been identified which are expressed in totipotent stem cells and gemmule cells. Furthermore, genes are described for the three main cell lineages in sponge, which share a common origin from archaeocytes and result in the differentiation of skeletal, epithelial, and contractile cells.

Arginine kinase in the demosponge Suberites domuncula:regulation of its expression and catalytic activity by silicic acid

In Demospongiae (phylum Porifera) the formation of the siliceous skeleton,composed of spicules, is an energetically expensive reaction. The present study demonstrates that primmorphs from the demosponge Suberites domuncula express the gene for arginine kinase after exposure to exogenous silicic acid. The deduced sponge arginine kinase sequence displays the two characteristic domains of the ATP:guanido phosphotransferases; it can be grouped to the `usual' mono-domain 40 kDa guanidino kinases (arginine kinases). Phylogenetic studies indicate that the metazoan guanidino kinases evolved from this ancestral sponge enzyme; among them are also the `unusual'two-domain 80 kDa guanidino kinases. The high expression level of the arginine kinase gene was already measurable 1 day after addition of silicic acid by northern blot, as well as by in situ hybridization analysis. Parallel determinations of enzyme activity confirmed that high levels of arginine kinase are present in primmorphs that had been exposed for 1-5 days to silicic acid. Finally, transmission electron-microscopical studies showed that primmorphs containing high levels of arginine kinase also produce siliceous spicules. These data highlight that silicic acid is an inorganic morphogenetic factor that induces the expression of the arginine kinase, which in turn probably catalyzes the reversible transfer of high-energy phosphoryl groups.

Bauplan of Urmetazoa: Basis for Genetic Complexity of Metazoa

Sponges were first grouped to the animal-plants or plant-animals then to the Zoophyta or Mesozoa and finally to the Parazoa. Only after the application of molecular biological techniques was it possible to place the Porifera monophyletically with the other metazoan phyla, justifying a unification of all multicellular animals to only one kingdom, the Metazoa. The first strong support came from the discovery that cell-cell and cell-matrix adhesion molecules that were cloned from sponges and were subsequently expressed share a high DNA sequence and protein function similarity with the corresponding molecules of other metazoans. Besides these evolutionary novelties for Metazoa, sponges also have morphogens and transcription factors in common with other metazoan phyla. Surprisingly, even those elements exist in Porifera, which are characteristic for pattern and axis formation. Recent studies showed that epithelial layers of sponges are sealed against the extracellular milieu through tight-junction proteins. The cell culture system from sponges, the primmorphs, was suitable for understanding morphogenetic events. Finally, stem cell marker genes were isolated, which underscored that sponge cells have the capacity to differentiate. In the relatively short period of time, approximately 200 million years, the basic pathways had to be established that allowed the transition for multicellular organisms to a colonial system through the formation of adhesion molecules; based on the development of a complex immune system and the apoptotic machinery of an integrated system, the Urmetazoa, which evolved approximately 800 million years ago, could be reached. Hence, the Bauplan of the hypothetical Urmetazoa can now be constructed according to genomic regulatory systems similar to those found in higher Metazoa. These data caused a paradigmatic change; the Porifera are complex and simple but by far not primitive.

The Chemokine Networks in Sponges: Potential Roles in Morphogenesis, Immunity and Stem Cell Formation

Porifera (sponges) are now well accepted as the phylum which branched off first from the common ancestor of all metazoans, the Urmetazoa. The transition to the Metazoa became possible because during this phase, cell-cell as well as cell-matrix adhesion molecules evolved which allowed the formation of a colonial stage of animals. The next prerequisite for the evolution to the Urmetazoa was the establishment of an effective immune system which, flanked by apoptosis, allowed the formation of a first level of individuation. In sponges (with the model Suberites domuncula and Geodia cydonium), the main mediators of the immune responses are the chemokines. Since sponges lack a vascular system and consequently blood cells (in the narrow sense), we have used the term chemokines (in a broad sense) to highlight that the complex network of intercellular mediators initiates besides differentiation processes also cell movement. In the present review, the cDNAs encoding the following chemokines were described and the roles of their deduced proteins during self-self and nonself recognition outlined: the allograft inflammatory factor, the glutathione peroxidase, the endothelial-monocyte-activating polypeptide, the pre-B-cell colony-enhancing factor and the myotrophin as well as an enzyme, the (2-5)A synthetase, which is involved in cytokine response in vertebrates. A further step required to reach the evolutionary step of the integrated stage of the Urmetazoa was the acquisition of a stem cell system. In this review, first markers for stem cells (mesenchymal stem cell-like protein) as well as for chemokines involved in the maintenance of stem cells (noggin and glia maturation factor) are described at the molecular level, and a first functional analysis is approached. Taken together, it is outlined that the chemokine network was essential for the establishment of metazoans, which evolved approximately 600 to 800 million years ago.