Isolation and cultivation of endosymbiotic algae from green hydra and phylogenetic analysis of 18S rDNA sequences

Symbiotic associations are of wide significance in evolution and biodiversity. The green hydra is a typical example of endosymbiosis. In its gastrodermal myoepithelial cells it harbors the individuals of a unicellular green algae. Endosymbiotic algae from green hydra have been successfully isolated and permanently maintained in a stable clean lab culture for the first time. We reconstructed the phylogeny of isolated endosymbiotic algae using the 18S rRNA gene to clarify its current status and to validate the traditional inclusion of these endosymbiotic algae within the Chlorella genus. Molecular analyses established that different genera and species of unicellular green algae could be present as symbionts in green hydra, depending on the natural habitat of a particular strain of green hydra.

Fluorescent nanocrystals reveal regulated portals of entry into and between the cells of Hydra

Initially viewed as innovative carriers for biomedical applications, with unique photophysical properties and great versatility to be decorated at their surface with suitable molecules, nanoparticles can also play active roles in mediating biological effects, suggesting the need to deeply investigate the mechanisms underlying cell-nanoparticle interaction and to identify the molecular players. Here we show that the cell uptake of fluorescent CdSe/CdS quantum rods (QRs) by Hydra vulgaris, a simple model organism at the base of metazoan evolution, can be tuned by modifying nanoparticle surface charge. At acidic pH, amino-PEG coated QRs, showing positive surface charge, are actively internalized by tentacle and body ectodermal cells, while negatively charged nanoparticles are not uptaken. In order to identify the molecular factors underlying QR uptake at acidic pH, we provide functional evidence of annexins involvement and explain the QR uptake as the combined result of QR positive charge and annexin membrane insertion. Moreover, tracking QR labelled cells during development and regeneration allowed us to uncover novel intercellular trafficking and cell dynamics underlying the remarkable plasticity of this ancient organism.

Models for the generation and interpretation of gradients

Source regions for morphogen gradients-organizing regions-can be generated if a local self-enhancing reaction is coupled with a long-ranging reaction that acts antagonistically. Resulting gradients can be translated into patterns of stable gene activities using genes whose products have a positive feedback on the activation on themselves. If several autoregulatory genes compete with each other for activity, cells make an unequivocal choice. Although the signal consists of a smoothly graded distribution, the all-or-nothing response of the cells leads to regions of differently determined cells that are delimited by sharp borders. In some systems, it is not the absolute but the relative level of a gradient that matters. The sequence of head, tentacles, and foot formation in hydra is controlled by a head activation gradient and is an example of this widely used but conceptually rather neglected mode. For subpatterns such as legs and wings, different "compartments" cooperate to produce new signaling substances. Here, morphogen production is restricted to the common borders or where they intersect. The model accounts for the formation of substructures in pairs at the correct positions within the embryo and for the correct orientation and handedness with respect to the main body axes.

Glutamatergic transmission in hydra: NMDA/D-serine affects the electrical activity of the body and tentacles of Hydra vulgaris (Cnidaria, Hydrozoa)

Previous electrophysiological studies on the early-evolved metazoan Hydra vulgaris provided evidence that glutamate, acting through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate receptors, affects hydra's pacemaker systems; immunocytochemical studies showed that N-methyl-d-aspartate (NMDA) receptors were present in hydra tentacles; behavioral studies demonstrated that NMDA/d-serine affected mouth opening induced by reduced glutathione, and with AMPA/kainate, discharge of nematocysts. In this study, extracellular recordings were made from the tentacle and peduncle of hydra during bath application of NMDA and d-serine (both at 1 x 10(-5) mol l(-1) to 1 x 10(-9) mol l(-1)) in the presence of 1 x 10(-7) mol l(-1) AMPA or kainate. NMDA/d-serine produced a significant increase in tentacle activity, increasing the rate of tentacle pacemaker pulses (TPs) at 1 x 10(-7) mol l(-1), and small, behaviorally uncorrelated tentacle pulses (SUTPs) at 1 x 10(-5) mol l(-1). The NMDA antagonist, d-2-amino-5-phosphonopentanoic acid (D-AP5), counteracted the effects. NMDA/d-serine (1 x 10(-7) mol l(-1)) also caused a potentially significant (trend) decrease in the rate of small, behaviorally uncorrelated electrical body pulses (SUBPs) and rhythmic potentials (RPs). The effect was counteracted by D-AP5. The ectodermal contraction burst (CB) pacemaker system was unaffected by NMDA/d-serine. Our results indicate that glutamate, acting on NMDA/AMPA-kainate receptors, may cause opposing effects on the coordinating systems of tentacle and body-exciting the tentacle effectors and potentially causing an inhibition in the body column.

Mechanogenetic coupling of Hydra symmetry breaking and driven Turing instability model

The freshwater polyp Hydra can regenerate from tissue fragments or random cell aggregates. We show that the axis-defining step ("symmetry breaking") of regeneration requires mechanical inflation-collapse oscillations of the initial cell ball. We present experimental evidence that axis definition is retarded if these oscillations are slowed down mechanically. When biochemical signaling related to axis formation is perturbed, the oscillation phase is extended and axis formation is retarded as well. We suggest that mechanical oscillations play a triggering role in axis definition. We extend earlier reaction-diffusion models for Hydra regrowth by coupling morphogen transport to mechanical stress caused by the oscillations. The modified reaction-diffusion model reproduces well two important experimental observations: 1), the existence of an optimum size for regeneration, and 2), the dependence of the symmetry breaking time on the properties of the mechanical oscillations.

PI3K and ERK 1-2 regulate early stages during head regeneration in hydra

Different signaling systems coordinate and regulate the development of a multicellular organism. In hydra, the canonical Wnt pathway and the signal transduction pathways mediated by PKC and Src regulate early stages of head formation. In this paper, we present evidence for the participation of a third pathway, the PI3K-PKB pathway, involved in this process. The data presented here are consistent with the participation of ERK 1-2 as a point of convergence for the transduction pathways mediated by PKC, Src and PI3K for the regulation of the regeneration of the head in hydra. The specific developmental point regulated by them appears to be the commitment of tissue at the apical end of the regenerate to form the head organizer.

Effects of norflurazon on green and brown hydra

For the first time effects of norflurazon on green (Hydra viridissima Pallas, 1766) and brown hydra (Hydra oligactis Pallas, 1766) in high- and low-light conditions were investigated in order to establish the extent of damage that this substance inflicts, with special emphasis on the "bleaching effect" and the effect on hydra-algae symbiosis. Green hydra is a typical example of an endosymbiotic organism. The gastrodermal myoepithelial cells of green hydra contain endosymbiotic algae. Norflurazon is a selective translocational herbicide that induces a "bleaching effect" on newly developed chloroplasts, resulting in a decrease of photosynthetic activity and viability of the organism. In the experiment, morphological (binocular), cytological and histological (Bouin fixative, dehydration, paraplast embedding, Hämalaun-eosine staining, light microscope) and conventional transmission electron microscopy (cTEM) (glutaraldehyde, dehydration, raisin, uranyl-acetate, Pb-citrate) were used. Depending on the concentration and light conditions, norflurazon caused mortality, deformations, changes in behavior, locomotion and asexual reproduction, changes in the structure of all 3 layers in hydras, changes in the position and shape of endosymbiotic algae inside the hydra body as well as ultrastructural changes of treated hydras and endosymbiotic algae. Under low concentrations of norflurazon the effects on hydra were similar to the controls, while in the highest concentrations especially manifested were antichloroplastal and antimitochondrial effects. Norflurazon caused a great extent of damage and induced deleterious effects also upon other cellular components such as cellular membranes, ER, Golgi apparatus, ribosomes. Newly developed buds in symbiotic green hydras were not bleached. After a recovery period, the green hydra individuals that had survived re-established regular endosymbiosis with algae and recovered completely, whereas brown hydras recovered only partially.

Rod-shaped nanocrystals elicit neuronal activity in vivo

The development of novel nanomaterials has raised great interest in efforts to evaluate their effect on biological systems, ranging from single cells to whole animals. In particular, there exists an open question regarding whether nanoparticles per se can elicit biological responses, which could interfere with the phenomena they are intended to measure. Here it is reported that challenging the small cnidaria Hydra vulgaris in vivo with rod-shaped semiconductor nanoparticles, also known as quantum rods (QRs), results in an unexpected tentacle-writhing behavior, which is Ca(2+) dependent and relies on the presence of tentacle neurons. Due to the absence of surface functionalization of the QRs with specific ligands, and considering that spherical nanoparticles with same composition as the QRs fail to induce any in vivo behavior on the same experimental model, it is suggested that unique shape-tunable electrical properties of the QRs may account for the neuronal stimulation. This model system may represent a widely applicable tool for screening neuronal response to nanoparticles in vivo.

The extracellular matrix of hydra is a porous sheet and contains type IV collagen

Hydra, as an early diploblastic metazoan, has a well-defined extracellular matrix (ECM) called mesoglea. It is organized in a tri-laminar pattern with one centrally located interstitial matrix that contains type I collagen and two sub-epithelial zones that resemble a basal lamina containing laminin and possibly type IV collagen. This study used monoclonal antibodies to the three hydra mesoglea components (type I, type IV collagens and laminin) and immunofluorescent staining to visualize hydra mesoglea structure and the relationship between these mesoglea components. In addition, hydra mesoglea was isolated free of cells and studied with immunofluorescence and scanning electron microscopy (SEM). Our results show that type IV collagen co-localizes with laminin in the basal lamina whereas type I collagen forms a grid pattern of fibers in the interstitial matrix. The isolated mesoglea can maintain its structural stability without epithelial cell attachment. Hydra mesoglea is porous with multiple trans-mesoglea pores ranging from 0.5 to 1 microm in diameter and about six pores per 100 microm(2) in density. We think these trans-mesoglea pores provide a structural base for epithelial cells on both sides to form multiple trans-mesoglea cell-cell contacts. Based on these findings, we propose a new model of hydra mesoglea structure.

Expulsion of symbiotic algae during feeding by the green hydra--a mechanism for regulating symbiont density?

Background

Algal-cnidarian symbiosis is one of the main factors contributing to the success of cnidarians, and is crucial for the maintenance of coral reefs. While loss of the symbionts (such as in coral bleaching) may cause the death of the cnidarian host, over-proliferation of the algae may also harm the host. Thus, there is a need for the host to regulate the population density of its symbionts. In the green hydra, Chlorohydra viridissima, the density of symbiotic algae may be controlled through host modulation of the algal cell cycle. Alternatively, Chlorohydra may actively expel their endosymbionts, although this phenomenon has only been observed under experimentally contrived stress conditions.

Principal Findings

We show, using light and electron microscopy, that Chlorohydra actively expel endosymbiotic algal cells during predatory feeding on Artemia. This expulsion occurs as part of the apocrine mode of secretion from the endodermal digestive cells, but may also occur via an independent exocytotic mechanism.

Significance

Our results demonstrate, for the first time, active expulsion of endosymbiotic algae from cnidarians under natural conditions. We suggest this phenomenon may represent a mechanism whereby cnidarians can expel excess symbiotic algae when an alternative form of nutrition is available in the form of prey.

Self-propelled particle model for cell-sorting phenomena

A self-propelled particle model is introduced to study cell sorting occurring in some living organisms. This allows us to evaluate the influence of intrinsic cell motility separately from differential adhesion with fluctuations, a mechanism previously shown to be sufficient to explain a variety of cell rearrangement processes. We find that the tendency of cells to actively follow their neighbors greatly reduces segregation time scales. A finite-size analysis of the sorting process reveals clear algebraic growth laws as in physical phase-ordering processes, albeit with unusual scaling exponents.

Neuropeptides and their functions in Hydra

In order to identify novel peptide signaling molecules involved in the regulation of developmental and physiological processes in the freshwater cnidarian, Hydra magnipapillata, we initiated a systematic peptide screening project, the Hydra Peptide Project. In the project, twelve neuropeptides were identified so far. The LWamide family is composed of seven members, which share a GLWamide motif at their C-termini. All the peptides have an ability to induce metamorphosis of Hydractinia serrata planula larvae into polyps. In Hydra, LWamides induce detachment of the bud from a parental polyp. A neuropeptide, Hym-355, enhances neuronal differentiation by inducing the multipotent interstitial stem cells to enter the neuron differentiation pathway. A myoactive neutopeptide, Hym-176, specifically and reversibly induces contraction of the ectodermal muscle of the body column, in particularly in the peduncle region of epithelial Hydra that totally lack nerve cells. Two members of a novel neuropeptide family (FRamides) were contained in the same precursor. However, they have opposite myoactive functions in epithelial hydra. From these results, it seems fair to say reasonable to conclude that the so-called 'primitive' nervous system of Hydra is in reality more complex than generally believed.

Remodeling the model organism: matrix metalloproteinase functions in invertebrates

The matrix metalloproteinase (MMP) family of extracellular proteases is conserved throughout the animal kingdom. Studies of invertebrate MMPs have demonstrated they are involved in tissue remodeling. In Drosophila, MMPs are required for histolysis, tracheal growth, tissue invasion, axon guidance, and dendritic remodeling. Recent work demonstrates that MMPs also participate in Drosophila tumor invasion. In Caenorhabditis elegans an MMP is involved in anchor cell invasion; a Hydra MMP is important for regeneration and maintaining cell identity; and a sea urchin MMP degrades matrix to allow hatching. In worms and in flies, MMPs are regulated by the JNK pathway.

[N-arachidonoyl dopamine is a possible factor of the rate of tentacle formation in freshwater hydra]

The effect of N-arachidonoyl dopamine, haloperidol, and their mixture on the rate of tentacle formation was studied during regeneration of the gastral and basal fragments of freshwater hydra. Some concentrations of haloperidol inhibited the tentacle formation, which was more pronounced in the basal fragment. N-arachidonoyl dopamine accelerated the tentacle formation in both fragments, particularly, in the basal one (an inversion of the natural difference in the rate of tentacle formation between the gastral and basal fragments). After the exposure to the mixture of these drugs, the effects of each of them were observed. Mass spectrometry assay has demonstrated endogenous N-arachidonoyl dopamine in the intact hydra homogenate. The possible involvement of this acyl-neurotransmitter in the regulation of the rate of tentacle formation in regenerating hydra is discussed.

Detection of expression patterns in Hydra pattern formation

Cnidarians are simple metazoans with only two body layers and a primitive nervous system. They are famous for their nearly indefinite regeneration capacity. Recent work has identified most of the Wnt subfamilies and Wnt antagonists known from vertebrates in this basal animal model. Wnt signaling and BMP signaling have been shown to act in Hydra pattern formation and regeneration. Because recent genomic work in Hydra and Nematostella revealed many genes for vertebrate signaling pathways and transcription factors to be present in this more than 500 Myr-year-old phylum, future work will focus on the function and expression of these genes in Hydra pattern formation and regeneration. This chapter presents an in situ hybridization protocol, which is largely based on a lab protocol of the Bode lab that has proven to be extremely useful in the characterization of many developmental genes from Hydra.

Hydra--ancient model with modern outfit

The control of growth and differentiation is a central question not only for developmental biologists but increasingly for medical research as well. The freshwater polyp hydra was one of the first organisms to be used as a model system for the study of this question. It was chosen because of its simple body plan and because it is made up of only seven to eight different cell types. Recent research has shown that despite their simple body plan, cnidarians already exhibit an impressive repertoire of molecular tools which are responsible for the control of growth and differentiation and amongst which peptides appear to play an important role.

Osmotically driven prey disintegration in the gastrovascular cavity of the green hydra by a pore‐forming protein

Pore-forming proteins (PFPs) are water-soluble proteins able to integrate into target membranes to form transmembrane pores. They are common determinants of bacterial pathogenicity and are often found in animal venoms. We recently isolated and characterized Hydralysins (Hlns), paralytic PFPs from the venomous green hydra Chlorohydra viridissima that are not found within the nematocytes, suggesting they are not involved in prey capture. The present study aimed to decipher the biological role of Hlns. Using in situ hybridization and immunohistochemistry, we show that Hlns are expressed by digestive cells surrounding the gastrovascular cavity (GVC) of Chlorohydra and secreted onto the prey during feeding. At biologically relevant concentrations, Hlns bind prey membranes and form pores, lysing the cells and disintegrating the prey tissue. Hlns are unable to bind Chlorohydra membranes, thus protecting the producing animal from the destructive effect of its own cytolytic protein. We suggest that osmotic disintegration of the prey within the GVC by Hlns, followed by phagocytosis and intracellular digestion, allows the soft-bodied green hydra to feed on hard, cuticle-covered prey while lacking the physical means to mechanically disintegrate it. Our results extend the biological significance of PFPs beyond the commonly expected offensive or defensive roles.

Unifying principles of regeneration I: Epimorphosis versus morphallaxis

Because research on regeneration has a long history, some classic definitions and concepts about regeneration which were established in earlier times have been retained without reconsideration for a long time, even though many relevant new findings have accumulated. To clarify the points on which research should be focused on for elucidating the mechanisms of regeneration, we should reconsider such classical definitions and principles of regeneration at the cellular and molecular level. Here, we consider two differing principles of regeneration which have been classically defined as 'epimorphosis' and 'morphallaxis', and propose the abandonment of these classical categories and their replacement by a new unifying principle in order to facilitate regeneration studies.

Suppression subtractive hybridization identifies novel transcripts in regenerating Hydra littoralis

Despite considerable interest in the biologic processes of regeneration and stem cell activation, little is known about the genes involved in these transformative events. In a Hydra littoralis model of regeneration, we employed a rapid shotgun suppression subtractive hybridization strategy to identify genes that are uniquely expressed in regenerating tissue. With an adaptor-PCR based technique, 16 candidate transcripts were identified, 15 were confirmed unique to mRNA isolated from hydra undergoing regeneration. Of these, 6 were undescribed in GenBank and allied expressed sequence tag (EST) databases (GenBank + EMBL + DDBJ + PDB and the Hydra EST database). BLAST analysis of these sequences identified remarkably similar sequences in anonymous ESTs found in a wide variety of animal species.

Synthesis and Biological Assay of GSH Functionalized Fluorescent Quantum Dots for Staining Hydra vulgaris

Quantum dots (QDs) have been used extensively as fluorescent markers in several studies on living cells. Here, we report the synthesis of conjugates based on glutathione (GSH) and QDs (GSH-QDs) and we prove how these functionalized fluorescent probes can be used for staining a freshwater invertebrate called Hydra vulgaris. GSH is known to promote Hydra feeding response by inducing mouth opening. We demonstrate that GSH-QDs as well are able to elicit biological activity in such an animal, which results in the fluorescent staining of Hydra. GSH-QDs, once they reach the gastric region, are internalized by endodermal cells. The efficiency of GSH-QD internalization increases significantly when nanoparticles are coadministrated with free GSH. We also compared the behavior of bare QDs to that of GSH-QDs both in the presence and in the absence of free GSH. The conclusions from these series of experiments point to the presence of GSH binding proteins in the endodermal cell layer and uncover a novel role played by glutathione in this organism.

Head regeneration in wild-type hydra requires de novo neurogenesis

Because head regeneration occurs in nerve-free hydra mutants, neurogenesis was regarded as dispensable for this process. Here, in wild-type hydra, we tested the function of the ParaHox gsx homolog gene, cnox-2,which is a specific marker for bipotent neuronal progenitors, expressed in cycling interstitial cells that give rise to apical neurons and gastric nematoblasts (i.e. sensory mechanoreceptor precursors). cnox-2 RNAi silencing leads to a dramatic downregulation of hyZic, prdl-a, gscand cnASH, whereas hyCOUP-TF is upregulated. cnox-2indeed acts as an upstream regulator of the neuronal and nematocyte differentiation pathways, as cnox-2(-) hydra display a drastic reduction in apical neurons and gastric nematoblasts, a disorganized apical nervous system and a decreased body size. During head regeneration, the locally restricted de novo neurogenesis that precedes head formation is cnox-2 dependent: cnox-2 expression is induced in neuronal precursors and differentiating neurons that appear in the regenerating tip; cnox-2 RNAi silencing reduces this de novo neurogenesis and delays head formation. Similarly, the disappearance of cnox-2+cells in sf-1 mutants also correlates with head regeneration blockade. Hence in wild-type hydra, head regeneration requires the cnox-2 neurogenic function. When neurogenesis is missing, an alternative, slower and less efficient, head developmental program is possibly activated.

The cAMP response element binding protein (CREB) as an integrative HUB selector in metazoans: Clues from the hydra model system

In eukaryotic cells, a multiplicity of extra-cellular signals can activate a unique signal transduction system that at the nuclear level will turn on a variety of target genes, eliciting thus diverse responses adapted to the initial signal. How distinct signals can converge on a unique signalling pathway that will nevertheless produce signal-specific responses provides a theoretical paradox that can be traced back early in evolution. In bilaterians, the CREB pathway connects diverse extra-cellular signals via cytoplasmic kinases to the CREB transcription factor and the CBP co-activator, regulating according to the context, cell survival, cell proliferation, cell differentiation, pro-apoptosis, long-term memory, hence achieving a "hub" function for cellular and developmental processes. In hydra, the CREB pathway is highly conserved and activated during early head regeneration through RSK-dependent CREB phosphorylation. We show here that the CREB transcription factor and the RSK kinase are co-expressed in all three hydra cell lineages including dividing interstitial stem cells, proliferating nematoblasts, proliferating spermatogonia and spermatocytes, differentiating and mature neurons as well as ectodermal and endodermal myoepithelial cells. In addition, CREB gene expression is specifically up-regulated during early regeneration and early budding. When the CREB function was chemically prevented, the early post-amputation induction of the HyBraI gene was no longer observed and head regeneration was stacked. Thus, in hydra, the CREB pathway appears already involved in multiple tasks, such as reactivation of developmental programs in an adult context, self-renewal of stem cells, proliferation of progenitors and neurogenesis. Consequently, the hub function played by the CREB pathway was established early in animal evolution and might have contributed to the formation of an efficient oral pole through the integration of the neurogenic and patterning functions.

Nerve ring of the hypostome in hydra: is it an origin of the central nervous system of bilaterian animals?

A hypothesis, 'the nerve ring in hydra shares a common origin with the central nervous system in bilaterian animals', is discussed in this review. The nerve ring of hydra is a ring of neurons whose neurites make a bundle running circumferentially around the hypostome just above the tentacle zone. This nervous structure has unique features in the hydra nervous system. It shows a tight association of neurons in contrast to the diffuse nerve net seen in other regions. It shows static developmental characters in contrast to the dynamic features of hydra nerve net present in other regions. Moreover, its structure and location are similar to the central nervous system (CNS) of other animals without a complex CNS such as nematodes and starfishes. Functions of the hydra nerve ring are also studied to test the hypothesis. The identified function is a crumpling of the tentacles, corresponding to the function of the inner nerve ring of hydrozoan jellyfish. The jellyfish nerve ring is considered to be a primitive central nervous system of radiates. Considering all the information available, the hypothesis is highly possible.

New observations on green hydra symbiosis

New observations on green hydra symbiosis are described. Herbicide norflurazon was chosen as a "trigger" for analysis of these observations. Green hydra (Hydra viridissima Pallas, 1766) is a typical example of endosymbiosis. In its gastrodermal myoeptihelial cells it contains individuals of Chlorella vulgaris Beij. (KESSLER & HUSS 1992). Ultrastructural changes were observed by means of TEM. The newly described morphological features of green hydra symbiosis included a widening of the perialgal space, missing symbiosomes and joining of the existing perialgal spaces. Also, on the basis of the newly described mechanisms, the recovery of green hydra after a period of intoxication was explained. The final result of the disturbed symbiosis between hydra and algae was the reassembly of the endosymbiosis in surviving individuals.