Self/nonself recognition in Cnidaria: contact to allogeneic tissue does not result in elimination of nonself cells in Hydra vulgaris

Invertebrate Immunity, Natural Transplantation Immunity, Somatic and Germ Cell Parasitism, and Transposon Defense

While the vertebrate immune system consists of innate and adaptive branches, invertebrates only have innate immunity. This feature makes them an ideal model system for studying the cellular and molecular mechanisms of innate immunity sensu stricto without reciprocal interferences from adaptive immunity. Although invertebrate immunity is evolutionarily older and a precursor of vertebrate immunity, it is far from simple. Despite lacking lymphocytes and functional immunoglobulin, the invertebrate immune system has many sophisticated mechanisms and features, such as long-term immune memory, which, for decades, have been exclusively attributed to adaptive immunity. In this review, we describe the cellular and molecular aspects of invertebrate immunity, including the epigenetic foundation of innate memory, the transgenerational inheritance of immunity, genetic immunity against invading transposons, the mechanisms of self-recognition, natural transplantation, and germ/somatic cell parasitism.

Cnidarian Immunity and the Repertoire of Defense Mechanisms in Anthozoans

Anthozoa is the most specious class of the phylum Cnidaria that is phylogenetically basal within the Metazoa. It is an interesting group for studying the evolution of mutualisms and immunity, for despite their morphological simplicity, Anthozoans are unexpectedly immunologically complex, with large genomes and gene families similar to those of the Bilateria. Evidence indicates that the Anthozoan innate immune system is not only involved in the disruption of harmful microorganisms, but is also crucial in structuring tissue-associated microbial communities that are essential components of the cnidarian holobiont and useful to the animal's health for several functions including metabolism, immune defense, development, and behavior. Here, we report on the current state of the art of Anthozoan immunity. Like other invertebrates, Anthozoans possess immune mechanisms based on self/non-self-recognition. Although lacking adaptive immunity, they use a diverse repertoire of immune receptor signaling pathways (PRRs) to recognize a broad array of conserved microorganism-associated molecular patterns (MAMP). The intracellular signaling cascades lead to gene transcription up to endpoints of release of molecules that kill the pathogens, defend the self by maintaining homeostasis, and modulate the wound repair process. The cells play a fundamental role in immunity, as they display phagocytic activities and secrete mucus, which acts as a physicochemical barrier preventing or slowing down the proliferation of potential invaders. Finally, we describe the current state of knowledge of some immune effectors in Anthozoan species, including the potential role of toxins and the inflammatory response in the Mediterranean Anthozoan Anemonia viridis following injection of various foreign particles differing in type and dimensions, including pathogenetic bacteria.

KEGG orthology-based annotation of the predicted proteome of Acropora digitifera: ZoophyteBase - an open access and searchable database of a coral genome

BACKGROUND

Contemporary coral reef research has firmly established that a genomic approach is urgently needed to better understand the effects of anthropogenic environmental stress and global climate change on coral holobiont interactions. Here we present KEGG orthology-based annotation of the complete genome sequence of the scleractinian coral Acropora digitifera and provide the first comprehensive view of the genome of a reef-building coral by applying advanced bioinformatics.

DESCRIPTION

Sequences from the KEGG database of protein function were used to construct hidden Markov models. These models were used to search the predicted proteome of A. digitifera to establish complete genomic annotation. The annotated dataset is published in ZoophyteBase, an open access format with different options for searching the data. A particularly useful feature is the ability to use a Google-like search engine that links query words to protein attributes. We present features of the annotation that underpin the molecular structure of key processes of coral physiology that include (1) regulatory proteins of symbiosis, (2) planula and early developmental proteins, (3) neural messengers, receptors and sensory proteins, (4) calcification and Ca2+-signalling proteins, (5) plant-derived proteins, (6) proteins of nitrogen metabolism, (7) DNA repair proteins, (8) stress response proteins, (9) antioxidant and redox-protective proteins, (10) proteins of cellular apoptosis, (11) microbial symbioses and pathogenicity proteins, (12) proteins of viral pathogenicity, (13) toxins and venom, (14) proteins of the chemical defensome and (15) coral epigenetics.

CONCLUSIONS

We advocate that providing annotation in an open-access searchable database available to the public domain will give an unprecedented foundation to interrogate the fundamental molecular structure and interactions of coral symbiosis and allow critical questions to be addressed at the genomic level based on combined aspects of evolutionary, developmental, metabolic, and environmental perspectives.

Apoptosis in pre-Bilaterians: Hydra as a model

Hydra is a member of the ancient metazoan phylum Cnidaria and is an especially well investigated model organism for questions of the evolutionary origin of metazoan processes. Apoptosis in Hydra is important for the regulation of cellular homeostasis under different conditions of nutrient supply. The molecular mechanisms leading to apoptosis in Hydra are surprisingly extensive and comparable to those in mammals. Genome wide sequence analysis has revealed the presence of large caspase and Bcl-2 families, the apoptotic protease activating factor (APAF-1), inhibitors of apoptotic proteases (IAPs) and components of a putative death receptor pathway. Regulation of apoptosis in Hydra may involve BH-3 only proteins and survival pathways, possibly including insulin signalling.

Cnidarian immunity: a tale of two barriers

The phylum Cnidariais one of the earliest branches in the animal tree of life providing crucial insights into the early evolution of immunity. The diversity in cnidarian life histories and habitats raises several important issues relating to immunity. First, in the absence of specific immune cells, cnidarians must have effective mechanisms to defend against microbial pathogens. Second, to maintain tissue integrity, colonial forms have to rely on their capacity of self/nonself discrimination to rapidly detect approaching allogeneic cells as foreign and to eliminate them. And third, since cnidarians are colonized by complex bacterial communities and in many cases are home to algal symbionts, successful growth means for cnidarians to be able to distinguish between beneficial symbionts and pathogenic intruders. The aim of this chapter is to review the experimental evidence for innate immune reactions in Cnidaria. We show that in these diploblastic animals consisting of only two cell layers; the epithelial cells are able to mediate all innate immune responses. The endodermal epithelium appears as a chemical barrier employing antimicrobial peptides while the ectodermal epithelium is a physicochemical barrier supported by a glycocalix. Microbial recognition is mediated by pattern recognition receptors such as Toll- and Nod-like receptors. Together, the data support the hypothesis that the establishment of epithelial barriers represents an important step in evolution of host defense in eumetazoan animals more than 600 million years ago.

Invertebrate allorecognition: the origins of histocompatibility

Alloimmune specificity and histocompatibility, driven by genetic polymorphism, are ancient determinants of self-/non-self-recognition. Recent molecular genetic evidence has revealed an allodeterminant in the cnidarian Hydractinia that consistently predicts histocompatibility reactions.

Parasites in Emsian-EifelianFavosites(Anthozoa, Tabulata) from the Holy Cross Mountains (Poland): changes of distribution within colony

Organisms of unknown biological affinities, assigned to the genusChaetosalpinx, are known to infest Palaeozoic tabulate corals and stromatoporoids. Analysis of distribution of these parasites, performed on Emsian–Eifelian material ofFavosites goldfussi(Anthozoa, Tabulata) from the Northern Region of the Holy Cross Mountains (Poland), shows that parasites were absent in the early astogenetical stages, and that during astogeny both the absolute number of parasites per colony and the number of parasites per polyp were increasing. The latter can reach 2.7 parasites per polyp. Preferred settling places are in corallite corners (junction of three individuals), but dense infestation also produced settlement in the corallite walls (between two individuals). Probable causes of the increase are insufficient protection by host's cnidae, insufficient immune system response, and parasite ability to adapt to the host's defences.

The p53 tumor suppressor-like protein nvp63 mediates selective germ cell death in the sea anemone Nematostella vectensis

Here we report the identification and molecular function of the p53 tumor suppressor-like protein nvp63 in a non-bilaterian animal, the starlet sea anemone Nematostella vectensis. So far, p53-like proteins had been found in bilaterians only. The evolutionary origin of p53-like proteins is highly disputed and primordial p53-like proteins are variably thought to protect somatic cells from genotoxic stress. Here we show that ultraviolet (UV) irradiation at low levels selectively induces programmed cell death in early gametes but not somatic cells of adult N. vectensis polyps. We demonstrate with RNA interference that nvp63 mediates this cell death in vivo. Nvp63 is the most archaic member of three p53-like proteins found in N. vectensis and in congruence with all known p53-like proteins, nvp63 binds to the vertebrate p53 DNA recognition sequence and activates target gene transcription in vitro. A transactivation inhibitory domain at its C-terminus with high homology to the vertebrate p63 may regulate nvp63 on a molecular level. The genotoxic stress induced and nvp63 mediated apoptosis in N. vectensis gametes reveals an evolutionary ancient germ cell protective pathway which relies on p63-like proteins and is conserved from cnidarians to vertebrates.

Programmed cell death in Hydra

Hydra is one of the simplest metazoans and thus an important model organism for studies on the evolution of developmental mechanisms in multi-cellular animals. In Hydra apoptosis is involved in the regulation of cell numbers in response to feeding, in regeneration and in the removal of non-self cells. It also participates in the maintenance of cellular homeostasis in germ cells. During oogenesis a special "arrested" apoptosis of nurse cells is observed. The morphology of apoptotic hydra cells is almost indistinguishable from apoptosis in higher animals and caspases as well as members of the Bcl-2 family participate in the process.