Preparation techniques for transmission electron microscopy of Hydra

Energy status of chironomid larvae (Diptera: Chironomidae) from high alpine rivers (Tyrol, Austria)

Chironomids (non-biting midges) inhabit almost every wet or semi-wet continental environment on Earth with probably 10,000 different species. Species occurrence and composition are undoubtedly limited by environmental harshness and food availability being reflected in their energy stores. Most animals store energy as glycogen and lipid. They enable the animals to survive adverse situations and to continue growth, development, and reproduction. This general statement is also true for insects and also particularly true for chironomid larvae. The rationale behind this research was, that probably any stress, any environmental burden, and any harmful influence increases the energy requirement of individual larvae depleting energy stores. We developed new methods to measure the glycogen and lipid content in small tissue samples. Here we show how to apply these methods to single chironomid larvae to demonstrate their energy stores. We compared different locations of the high Alpine rivers along harshness gradient densely populated and dominated by chironomid larvae. All samples show very low energy stores without any major differences. We found glycogen concentrations below 0.01% of dry weight (DW) and lipid concentrations below 5% of DW irrespective of the specific sampling point. These values are among the lowest ever observed in chironomid larvae. We demonstrate that individuals living in extreme environment are stressed leading to reduced energy stores in their bodies. This appears to be a general feature of high altitude regions. Our results provide new insights and a better understanding of population and ecological dynamics in harsh mountainous areas, also in view of a changing climate.

Non-muscle myosin II drives critical steps of nematocyst morphogenesis

Nematocysts are generated by secretion of proteins into a post-Golgi compartment. They consist of a capsule that elongates into a long tube, which is coiled inside the capsule matrix and expelled during its nano-second discharge deployed for prey capture. The driving force for discharge is an extreme osmotic pressure of 150 bar. The complex processes of tube elongation and invagination under these biomechanical constraints have so far been elusive. Here, we show that a non-muscle myosin II homolog (HyNMII) is essential for nematocyst formation in Hydra. In early nematocysts, HyNMII assembles to a collar around the neck of the protruding tube. HyNMII then facilitates tube outgrowth by compressing it along the longitudinal axis as evidenced by inhibitor treatment and genetic knockdown. In addition, live imaging of a NOWA::NOWA-GFP transgenic line, which re-defined NOWA as a tube component facilitating invagination, allowed us to analyze the impact of HyNMII on tube maturation.

The Involvement of Cell-Type-Specific Glycans in Hydra Temporary Adhesion Revealed by a Lectin Screen

Hydra is a freshwater solitary polyp, capable of temporary adhesion to underwater surfaces. The reversible attachment is based on an adhesive material that is secreted from its basal disc cells and left behind on the substrate as a footprint. Despite Hydra constituting a standard model system in stem cell biology and tissue regeneration, few studies have addressed its bioadhesion. This project aimed to characterize the glycan composition of the Hydra adhesive, using a set of 23 commercially available lectins to label Hydra cells and footprints. The results indicated the presence of N-acetylglucosamine, N-acetylgalactosamine, fucose, and mannose in the adhesive material. The labeling revealed a meshwork-like substructure in the footprints, implying that the adhesive is mainly formed by fibers. Furthermore, lectins might serve as a marker for Hydra cells and structures, e.g., many labeled as glycan-rich nematocytes. Additionally, some unexpected patterns were uncovered, such as structures associated with radial muscle fibers and endodermal gland cells in the hypostome of developing buds.

The Hippo pathway regulates axis formation and morphogenesis in Hydra

How did cells of early metazoan organisms first organize themselves to form a body axis? The canonical Wnt pathway has been shown to be sufficient for induction of axis in Cnidaria, a sister group to Bilateria, and is important in bilaterian axis formation. Here, we provide experimental evidence that in cnidarian Hydra the Hippo pathway regulates the formation of a new axis during budding upstream of the Wnt pathway. The transcriptional target of the Hippo pathway, the transcriptional coactivator YAP, inhibits the initiation of budding in Hydra and is regulated by Hydra LATS. In addition, we show functions of the Hippo pathway in regulation of actin organization and cell proliferation in Hydra. We hypothesize that the Hippo pathway served as a link between continuous cell division, cell density, and axis formation early in metazoan evolution.

Measuring lysosomal size and frequency by electron microscopy

Changes in size and abundance of late endocytic and autophagic organelles are increasingly appreciated as highly indicative of the physiological or pathological conditions of cells. Electron microscopy (EM) is unsurpassed in high-resolution imaging of both ultrastructural and immunocytochemical features of subcellular compartments. EM-based morphometry permits precise quantitative analyses of organelles, especially after state-of-the-art cryopreparation. Here described step-by-step protocols cover (i) different approaches for sample preparation of almost any specimen, (ii) tools to identify and characterize classes or subpopulations of lysosomes and related organelles, and (iii) convenient, straightforward ways for manual, thus, non-automated measurements of globular or spheroid-shaped organelles.

Dynamic interactions within the host-associated microbiota cause tumor formation in the basal metazoan Hydra

The extent to which disturbances in the resident microbiota can compromise an animal’s health is poorly understood. Hydra is one of the evolutionary oldest animals with naturally occurring tumors. Here, we found a causal relationship between an environmental spirochete (Turneriella spec.) and tumorigenesis in Hydra. Unexpectedly, virulence of this pathogen requires the presence of Pseudomonas spec., a member of Hydra´s beneficial microbiome indicating that dynamic interactions between a resident bacterium and a pathogen cause tumor formation. The observation points to the crucial role of commensal bacteria in maintaining tissue homeostasis and adds support to the view that microbial community interactions are essential for disease. These findings in an organism that shares deep evolutionary connections with all animals have implications for our understanding of cancer.

Here we follow up on our initial observation of tumor formation in the basal metazoan Hydra and demonstrate that tumor development in one of the evolutionary oldest animals is caused by a dynamic interplay between an environmental spirochete, the host-associated resident microbiota, and the tissue homeostasis within the animal. Unexpectedly, the pathogenicity of the environmental bacterium Turneriella is context-dependent: the virulence of this pathogen requires the presence of a member of Hydra’s beneficial microbiome—the Pseudomonas bacterium. Dynamic interactions between two microbiota members have profound effects onto the host tissue homeostasis and fitness. Our data provide direct evidence for the important role of the resident microbiome in maintaining tissue homeostasis and pathogen defense, a fundamental process that is likely to take place in every tissue of every animal species. In summary, our study uncovers an evolutionary conserved role of the resident microbiome in guarding host’s tissue homeostasis.

The Origin of Mucosal Immunity: Lessons from the Holobiont Hydra

Historically, mucosal immunity—i.e., the portion of the immune system that protects an organism’s various mucous membranes from invasion by potentially pathogenic microbes—has been studied in single-cell epithelia in the gastrointestinal and upper respiratory tracts of vertebrates. Phylogenetically, mucosal surfaces appeared for the first time about 560 million years ago in members of the phylum Cnidaria. There are remarkable similarities and shared functions of mucosal immunity in vertebrates and innate immunity in cnidarians, such as Hydra species. Here, we propose a common origin for both systems and review observations that indicate that the ultimately simple holobiont Hydra provides both a new perspective on the relationship between bacteria and animal cells and a new prism for viewing the emergence and evolution of epithelial tissue-based innate immunity. In addition, recent breakthroughs in our understanding of immune responses in Hydra polyps reared under defined short-term gnotobiotic conditions open up the potential of Hydra as an animal research model for the study of common mucosal disorders.

A piece of the pi(e): The diverse roles of animal piRNAs and their PIWI partners

Small non-coding RNAs are indispensable to many biological processes. A class of endogenous small RNAs, termed PIWI-interacting RNAs (piRNAs) because of their association with PIWI proteins, has known roles in safeguarding the genome against inordinate transposon mobilization, embryonic development, and stem cell regulation, among others. This review discusses the biogenesis of animal piRNAs and their diverse functions together with their PIWI protein partners, both in the germline and in somatic cells, and highlights the evolutionarily conserved aspects of these molecular players in animal biology.

Bacteria-bacteria interactions within the microbiota of the ancestral metazoan Hydra contribute to fungal resistance

Epithelial surfaces of most animals are colonized by diverse microbial communities. Although it is generally agreed that commensal bacteria can serve beneficial functions, the processes involved are poorly understood. Here we report that in the basal metazoan Hydra, ectodermal epithelial cells are covered with a multilayered glycocalyx that provides a habitat for a distinctive microbial community. Removing this epithelial microbiota results in lethal infection by the filamentous fungus Fusarium sp. Restoring the complex microbiota in gnotobiotic polyps prevents pathogen infection. Although mono-associations with distinct members of the microbiota fail to provide full protection, additive and synergistic interactions of commensal bacteria are contributing to full fungal resistance. Our results highlight the importance of resident microbiota diversity as a protective factor against pathogen infections. Besides revealing insights into the in vivo function of commensal microbes in Hydra, our findings indicate that interactions among commensal bacteria are essential to inhibit pathogen infection.

The synaptonemal complex of basal metazoan hydra: more similarities to vertebrate than invertebrate meiosis model organisms

The synaptonemal complex (SC) is an evolutionarily well-conserved structure that mediates chromosome synapsis during prophase of the first meiotic division. Although its structure is conserved, the characterized protein components in the current metazoan meiosis model systems (Drosophila melanogaster, Caenorhabditis elegans, and Mus musculus) show no sequence homology, challenging the question of a single evolutionary origin of the SC. However, our recent studies revealed the monophyletic origin of the mammalian SC protein components. Many of them being ancient in Metazoa and already present in the cnidarian Hydra. Remarkably, a comparison between different model systems disclosed a great similarity between the SC components of Hydra and mammals while the proteins of the ecdysozoan systems (D. melanogaster and C. elegans) differ significantly. In this review, we introduce the basal-branching metazoan species Hydra as a potential novel invertebrate model system for meiosis research and particularly for the investigation of SC evolution, function and assembly. Also, available methods for SC research in Hydra are summarized.

Analysis of Hydra PIWI proteins and piRNAs uncover early evolutionary origins of the piRNA pathway

To preserve genome integrity, an evolutionarily conserved small RNA-based silencing mechanism involving PIWI proteins and PIWI-interacting RNAs (piRNAs) represses potentially deleterious transposons in animals. Although there has been extensive research into PIWI proteins in bilaterians, these proteins remain to be examined in ancient phyla. Here, we investigated the PIWI proteins Hywi and Hyli in the cnidarian Hydra, and found that both PIWI proteins are enriched in multipotent stem cells, germline stem cells, and in the female germline. Hywi and Hyli localize to the nuage, a perinuclear organelle that has been implicated in piRNA-mediated transposon silencing, together with other conserved nuage and piRNA pathway components. Our findings provide the first report of nuage protein localization patterns in a non-bilaterian. Hydra PIWI proteins possess symmetrical dimethylarginines: modified residues that are known to aid in PIWI protein localization to the nuage and proper piRNA loading. piRNA profiling suggests that transposons are the major targets of the piRNA pathway in Hydra. Our data suggest that piRNA biogenesis through the ping-pong amplification cycle occurs in Hydra and that Hywi and Hyli are likely to preferentially bind primary and secondary piRNAs, respectively. Presumptive piRNA clusters are unidirectionally transcribed and primarily give rise to piRNAs that are antisense to transposons. These results indicate that various conserved features of PIWI proteins, the piRNA pathway, and their associations with the nuage were likely established before the evolution of bilaterians.

Imaging inward and outward trafficking of gold nanoparticles in whole animals

Gold nanoparticles have emerged as novel safe and biocompatible tools for manifold applications, including biological imaging, clinical diagnostics, and therapeutics. The understanding of the mechanisms governing their interaction with living systems may help the design and development of new platforms for nanomedicine. Here we characterized the dynamics and kinetics of the events underlying the interaction of gold nanoparticles with a living organism, from the first interaction nanoparticle/cell membrane, to the intracellular trafficking and final extracellular clearance. By treating a simple water invertebrate (the cnidarian Hydra polyp) with functionalized gold nanoparticles, multiple inward and outward routes were imaged by ultrastructural analyses, including exosomes as novel undescribed carriers to shuttle the nanoparticles in and out the cells. From the time course imaging a highly dynamic picture emerged in which nanoparticles are rapidly internalized (from 30 min onward), recruited into vacuoles/endosome (24 h onward), which then fuse, compact and sort out the internalized material either to storage vacuoles or to late-endosome/lysosomes, determining almost complete clearance within 48 h from challenging. Beside classical routes, new portals of entry/exit were captured, including exosome-like structures as novel undescribed nanoparticle shuttles. The conservation of the endocytic/secretory machinery through evolution extends the value of our finding to mammalian systems providing dynamics and kinetics clues to take into account when designing nanomaterials to interface with biological entities.

Horizontal gene transfer contributed to the evolution of extracellular surface structures: the freshwater polyp Hydra is covered by a complex fibrous cuticle containing glycosaminoglycans and proteins of the PPOD and SWT (sweet tooth) families

The single-cell layered ectoderm of the fresh water polyp Hydra fulfills the function of an epidermis by protecting the animals from the surrounding medium. Its outer surface is covered by a fibrous structure termed the cuticle layer, with similarity to the extracellular surface coats of mammalian epithelia. In this paper we have identified molecular components of the cuticle. We show that its outermost layer contains glycoproteins and glycosaminoglycans and we have identified chondroitin and chondroitin-6-sulfate chains. In a search for proteins that could be involved in organising this structure we found PPOD proteins and several members of a protein family containing only SWT (sweet tooth) domains. Structural analyses indicate that PPODs consist of two tandem β-trefoil domains with similarity to carbohydrate-binding sites found in lectins. Experimental evidence confirmed that PPODs can bind sulfated glycans and are secreted into the cuticle layer from granules localized under the apical surface of the ectodermal epithelial cells. PPODs are taxon-specific proteins which appear to have entered the Hydra genome by horizontal gene transfer from bacteria. Their acquisition at the time Hydra evolved from a marine ancestor may have been critical for the transition to the freshwater environment.

The nervous system of Isodiametra pulchra (Acoela) with a discussion on the neuroanatomy of the Xenacoelomorpha and its evolutionary implications

Introduction

Acoels are microscopic marine worms that have become the focus of renewed debate and research due to their placement at the base of the Bilateria by molecular phylogenies. To date, Isodiametra pulchra is the most promising “model acoel” as it can be cultured and gene knockdown can be performed with double-stranded RNA. Despite its well-known morphology data on the nervous system are scarce. Therefore we examined this organ using various microscopic techniques, including histology, conventional histochemistry, electron microscopy, and immunocytochemistry in combination with CLSM and discuss our results in light of recently established phylogenies.

Results

The nervous system of Isodiametra pulchra consists of a bilobed brain with a dorsal posterior commissure, a frontal ring and tracts, four pairs of longitudinal neurite bundles, as well as a supramuscular and submuscular plexus. Serotonin-like immunoreactivity (SLI) is displayed in parts of the brain, the longitudinal neurite bundles and a large part of the supramuscular plexus, while FMRFamide-like immunoreactivity (RFLI) is displayed in parts of the brain and a distinct set of neurons, the longitudinal neurite bundles and the submuscular plexus. Despite this overlap SLI and RFLI are never colocalized. Most remarkable though is the presence of a distinct functional neuro-muscular system consisting of the statocyst, tracts, motor neurons and inner muscles, as well as the presence of various muscles that differ with regard to their ultrastructure and innervation.

Conclusions

The nervous system of Isodiametra pulchra consists of an insunk, bilobed brain, a peripheral part for perception and innervation of the smooth body-wall musculature as well as tracts and motor neurons that together with pseudostriated inner muscles are responsible for steering and quick movements. The insunk, bilobed brains with two to three commissures found in numerous acoels are homologous and evolved from a ring-commissural brain that was present in the stem species of acoelomorphs. The acoelomorph brain is bipartite, consisting of a Six3/6-dependend animal pole nervous system that persists throughout adulthood and an axial nervous system that does not develop by exhibiting a staggered pattern of conserved regulatory genes as in other bilaterians but by a nested pattern of these genes. This indicates that acoelomorphs stem from an ancestor with a simple brain or with a biphasic life cycle.