A novel neuropeptide, Hym-355, positively regulates neuron differentiation in Hydra

Thypedin, the multi copy precursor for the hydra peptide pedin, is a β-thymosin repeat-like domain containing protein

Pedin, a peptide of 13 amino acids, stimulates foot formation in hydra, one of the simplest metazoan animals. Here, we show that the corresponding transcripts are 3.8 kb in size encoding a precursor protein with a size of about 110 kDa, which contains 13 copies of the peptide. Interestingly, the deduced amino acid sequence of the precursor comprises 27 copies of a beta-thymosin-like repeat domain. Hence, we named the precursor protein thypedin. Pedin transcripts are present along the body axis of the animal with slightly higher abundance in the foot to bud region and in the head. Pedin is expressed mainly in epithelial cells of the ectoderm and endoderm. During budding it is present in the evaginating bud. The early appearance of transcripts during phases of cell-fate specification like budding indicates that pedin may be involved in differentiation processes in hydra. This is confirmed by the fact that pedin stimulates bud outgrowth. Thymosin-repeat containing proteins are well known for their regulatory influence on actin polymerisation. Here we show the first indirect evidence that thypedin may be able to interact with actin as well. Since actin polymerisation and depolymerisation processes are known to take place during morphogenetic processes, these findings may hint at new aspects of the function of pedin and its precursor in pattern formation in hydra.

Neuronal evolution: analysis of regulatory genes in a first-evolved nervous system, the hydra nervous system

Cnidarians represent the first animal phylum with an organized nervous system and a complex active behavior. The hydra nervous system is formed of sensory-motoneurons, ganglia neurons and mechanoreceptor cells named nematocytes, which all differentiate from a common stem cell. The neurons are organized as a nerve net and a subset of neurons participate in a more complex structure, the nerve ring that was identified in most cnidarian species at the base of the tentacles. In order to better understand the genetic control of this neuronal network, we analysed the expression of evolutionarily conserved regulatory genes in the hydra nervous system. The Prd-class homeogene prdl-b and the nuclear orphan receptor hyCOUP-TF are expressed at strong levels in proliferating nematoblasts, a lineage where they were found repressed during patterning and morphogenesis, and at low levels in distinct subsets of neurons. Interestingly, Prd-class homeobox and COUP-TF genes are also expressed during neurogenesis in bilaterians, suggesting that mechanoreceptor and neuronal cells derive from a common ancestral cell. Moreover, the Prd-class homeobox gene prdl-a, the Antp-class homeobox gene msh, and the thrombospondin-related gene TSP1, which are expressed in distinct subset of neurons in the adult polyp, are also expressed during early budding and/or head regeneration. These data strengthen the fact that two distinct regulations, one for neurogenesis and another for patterning, already apply to these regulatory genes, a feature also identified in bilaterian related genes.

Inhibition of metamorphosis by RFamide neuropeptides in planula larvae of Hydractinia echinata

The primitive nervous system in planula larvae of Hydractinia echinata (Cnidaria) has sensory neurons containing LWamide or RFamide neuropeptides. LWamides have been shown to induce metamorphosis of planula larvae into adult polyps. We report here that RFamides act antagonistically to LWamides. RFamides inhibit metamorphosis when applied to planula larvae during metamorphosis induction by treatment with LWamides (or other inducing agents such as CsCl ions, diacylglycerol and bacterial inducers). Our results show further that RFamides act downstream of LWamide release, presumably directly on target cells mediating metamorphosis. These observations support a model in which metamorphosis in H. echinata is regulated by sensory neurons secreting LWamides and RFamides in response to environmental cues.

Identification of a new member of the GLWamide peptide family: physiological activity and cellular localization in cnidarian polyps

KPNAYKGKLPIGLWamide, a novel member of the GLWamide peptide family, was isolated from Hydra magnipapillata. The purification was monitored with a bioassay: contraction of the retractor muscle of a sea anemone, Anthopleura fuscoviridis. The new peptide, termed Hym-370, is longer than the other GLWamides previously isolated from H. magnipapillata and another sea anemone, A. elegantissima. The amino acid sequence of Hym-370 is six residues longer at its N-terminal than a putative sequence previously deduced from the cDNA encoding the precursor protein. The new longer isoform, like the shorter GLWamides, evoked concentration-dependent muscle contractions in both H. magnipapillata and A. fuscoviridis. In contrast, Hym-248, one of the shorter GLWamide peptides, specifically induced contraction of the endodermal muscles in H. magnipapillata. This is the first case in which a member of the hydra GLWamide family (Hym-GLWamides) has exhibited an activity not shared by the others. Polyclonal antibodies were raised to the common C-terminal tripeptide GLWamide and were used in immunohistochemistry to localize the GLWamides in the tissue of two species of hydra, H. magnipapillata and H. oligactis, and one species of sea anemone, A. fuscoviridis. In each case, nerve cells were specifically labeled. These results suggest that the GLWamides are ubiquitous among cnidarians and are involved in regulating the excitability of specific muscles.

Transcription and translation inhibitors permit metamorphosis up to radiole formation in the serpulid polychaete Hydroides elegans haswell

Settlement and metamorphosis in most well-studied marine invertebrates are rapid processes, triggered by external cues. How this initial environmentally mediated response is transduced into morphogenetic events that culminate in the formation of a functional juvenile is still not well understood for any marine invertebrate. The response of larvae of the serpulid polychaete Hydroides elegans to inhibitors of mRNA and protein synthesis was examined to determine if metamorphosis requires these molecular processes. Competent larvae of H. elegans were induced to metamorphose by exposing them to a bacterial film or a 3-h pulse of 10 mM CsCl in the presence of the gene-transcription inhibitor DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole) or the translation inhibitor emetine. When induced to metamorphose in the presence of either inhibitor, larvae of H. elegans progressed through metamorphosis to the point at which branchial radioles start to develop. DRB and emetine inhibited the incorporation of radiolabeled uridine into RNA and radiolabeled methionine into peptides, respectively, indicating that they were effective in blocking the appropriate syntheses. Taken together, these results indicate that the induction of metamorphosis in H. elegans does not require de novo transcription or translation, and that the form of the juvenile worm is achieved in two phases. During the first phase, larvae respond to the inducer by attaching to the substratum, secreting a primary tube, resorbing the prototroch cilia, undergoing caudal elongation, and differentiating the collar; once the collar is formed, they begin secreting the secondary, calcified tube. During the second phase, the small worm develops branchial radioles and begins to grow, requiring new mRNA and protein syntheses.

Enhanced antibacterial activity in Hydra polyps lacking nerve cells

The nervous system evolved within cnidarians. When assessing antibacterial activity in the freshwater polyp Hydra, we observed a strong correlation between the number of neurons present and the antibacterial activity. Tissue lacking neurons had a drastically enhanced antibacterial activity against Gram-positive (Bacillus subtilis) and Gram-negative (E. coli) bacteria compared to control tissue. The results indicate direct and strong neural influences on immunity in the phylogenetically oldest animals having a nervous system.

Identification of a vasopressin-like immunoreactive substance in hydra

Vasopressin (VP)-like immunoreactivity has long been known in the hydra nervous system, but has not yet been structurally identified. In this study, using HPLC fractionation and an immunological assay, we have purified two peptides, FPQSFLPRGamide and SFLPRGamide, from Hydra magnipapillata. Both the peptides shared the same C-terminal structure, -PRGamide, with Arg-VP. The nonapeptide proved to be Hym-355, a peptide that stimulates neuronal differentiation in hydra. Detailed evaluation by competitive enzyme-linked immunosorbent assay (ELISA) and double immunostaining using anti-VP and anti-Hym-355 antibodies enabled us to conclude that the two peptides account for a major part of the VP-like immunoreactivity in hydra nerve cells.

Neuropeptides in cnidarians

Cnidarians are the lowest animal group having a nervous system. In the primitive nervous systems of cnidarians, peptides play important roles as neurotransmitters or neurohormones. So far, we have isolated and sequenced about 35 neuropeptides from different cnidarian classes (Hydrozoa, Scyphozoa, Anthozoa). All these neuropeptides have a C-terminal amide group, which protects against C-terminal degradation, but which also is important for receptor recognition. Also the N-termini of the cnidarian neuropeptides often contain different kinds of protecting groups (such as <Glu residues, L-3-phenyllactyl groups, and X-Pro or X-Pro-Pro sequences). Cnidarian neuropeptides are located in neuronal dense-core vesicles and are synthesized as preprohormones, which can contain up to 41 copies of a neuro peptide sequence. From Hydra, six different neuropeptide genes have been cloned so far. Each gene is expressed by a specific population of neurons, but in two instances coexpression of neuropeptide genes has been found. We have also cloned some of the cnidarian prohormone processing enzymes, among them the enzymes necessary for C-terminal amidation. These enzymes are closely related to their mammalian counterparts. All these data show that the primitive nervous systems of cnidarians have already acquired some of the sophisticated principles that we know from higher animals.

Developmental neurobiology of hydra, a model animal of cnidarians

Hydra belongs to the class Hydrozoa in the phylum Cnidaria. Hydra is a model animal whose cellular and developmental data are the most abundant among cnidarians. Hence, I discuss the developmental neurobiology of hydra. The hydra nerve net is a mosaic of neural subsets expressing a specific neural phenotype. The developmental dynamics of the nerve cells are unique. Neurons are produced continuously by differentiation from interstitial multipotent stem cells. These neurons are continuously displaced outwards along with epithelial cells and are sloughed off at the extremities. However, the spatial distribution of each neural subset is maintained. Mechanisms related to these phenomena, i.e., the position-dependent changes in neural phenotypes, are proposed. Nerve-net formation in hydra can be examined in various experimental systems. The conditions of nerve-net formation vary among the systems, so we can clarify the control factors at the cellular level by comparing nerve-net formation in different systems. By large-scale screening of peptide signal molecules, peptide molecules related to nerve-cell differentiation have been identified. The LPW family, composed of four members sharing common N-terminal L(or I)PW, inhibits nerve-cell differentiation in hydra. In contrast, Hym355 (FPQSFLPRG-NH3) activates nerve differentiation in hydra. LPWs are epitheliopeptides, whereas Hym355 is a neuropeptide. In the hypostome of hydra, a unique neuronal structure, the nerve ring, is observed. This structure shows the nerve association of neurites. Exceptionally, the tissue containing the nerve ring shows no tissue displacement during the tissue flow that involves the whole body. The neurons in the nerve ring show little turnover, although nerve cells in all other regions turn over continuously. These associations and quiet dynamics lead me to think that the nerve ring has features similar to those of the central nervous system in higher animals.

Developmental signaling in Hydra: what does it take to build a "simple" animal?

Developmental processes in multicellular animals depend on an array of signal transduction pathways. Studies of model organisms have identified a number of such pathways and dissected them in detail. However, these model organisms are all bilaterians. Investigations of the roles of signal transduction pathways in the early-diverging metazoan Hydra have revealed that a number of the well-known developmental signaling pathways were already in place in the last common ancestor of Hydra and bilaterians. In addition to these shared pathways, it appears that developmental processes in Hydra make use of pathways involving a variety of peptides. Such pathways have not yet been identified as developmental regulators in more recently diverged animals. In this review I will summarize work to date on developmental signaling pathways in Hydra and discuss the future directions in which such work will need to proceed to realize the potential that lies in this simple animal.

A possible role for the cnidarian homologue of serum response factor in decision making by undifferentiated cells

We have isolated the serum response factor (SRF) homologue from two hydrozoans, the freshwater polyp Hydra vulgaris and the marine colonial Hydractinia echinata; we have termed the Hydra gene HvSRF and the Hydractinia gene HeSRF. The MADS-box of both genes is identical in sequence and more similar to SRFs of other organisms than to non-SRF MADS-box-containing proteins from other organisms. Within the N terminus of the predicted protein, a motif of 14 amino acids is nearly identical between Hydra and Hydractinia. This motif is absent from other known SRF sequences. In the adult Hydra polyp, SRF is predominantly expressed in cells of the interstitial cell (I-cell) lineage. Expression of SRF ceases when I-cells differentiate into nerve cells, nematocytes, or gland cells. In the course of sexual reproduction in Hydractinia, SRF is expressed in female germ cells. During embryogenesis, SRF transcripts are observed in all blastomeres. Later on, SRF expression is turned off in cells forming the ectodermal layer but further on is expressed in cells of the central cell mass, from which the endodermal epithelial cells and the I-cell lineage originate. Expression eventually becomes restricted to the I-cell lineage. We conclude that hydrozoan SRF is expressed in all these cells, which still have the property for differentiation. In adult Hydra, the abundance of SRF transcripts varies during the day. The pacemaker of this diurnal rhythm is the feeding regime. HvSRF expression decreases by 4 h after feeding and returns to the initial level 12 h after feeding. When feeding is stopped, the cycle of SRF expression persists through the first day when the animals are not fed. It has been shown that feeding partly synchronizes the cell cycle of the epithelial cells but not that of the I-cells. We suggest that the epithelial cells affect SRF expression in I-cells and thereby influence the decision of I-cells to enter a differentiation pathway.

The foot formation stimulating peptide pedibin is also involved in patterning of the head in hydra

Pedibin, a peptide of 21 amino acids, has been shown to stimulate foot formation in hydra, one of the simplest metazoan animals. The data presented here show that pedibin is synthesized as a precursor of 49 amino acids. A putative cleavage site precedes the peptide as purified from hydra tissue. The precursor, like pedibin, accelerates foot regeneration. Pedibin transcripts are concentrated in the foot region of hydra as expected, but are also present in the head region accumulating in the tentacle bases. The early appearance of pedibin transcripts during phases of cell fate specification like budding and regeneration implies that in hydra, pedibin plays an important role in patterning processes of foot and head. This is confirmed by the finding that pedibin also stimulates bud outgrowth.

Polyps, peptides and patterning

Peptides serve as important signalling molecules in development and differentiation in the simple metazoan Hydra. A systematic approach (The Hydra Peptide Project) has revealed that Hydra contains several hundreds of peptide signalling molecules, some of which are neuropeptides and others emanate from epithelial cells. These peptides control biological processes as diverse as muscle contraction, neuron differentiation, and the positional value gradient. Signal peptides cause changes in cell behaviour by controlling target genes such as matrix metalloproteases. The abundance of peptides in Hydra raises the question of whether, in early metazoan evolution, cell-cell communication was based mainly on these small molecules rather than on the growth-factor-like cytokines that control differentiation and development in higher animals.

Evolving better brains: a need for neurotrophins?

The NGF family of neurotrophins has a crucial role in regulating neuron numbers during vertebrate development. Six years ago the prediction was made that invertebrates with simple nervous systems, such as Caenorhabditis elegans, would lack neurotrophins. Surprisingly, it now appears that not only C. elegans but also Drosophila melanogaster, lack homologs of the neurotrophins or their trk receptors. Furthermore, functional studies indicate that control of neuronal numbers in Drosophila is primarily dependent on steroids. By contrast, a recognizable trk homolog exists in molluscs, a phylum that includes species with the most complex nervous systems in the invertebrate kingdom. This suggests that neurotrophic signaling mechanisms might be one of the prerequisites for evolution of complex nervous systems. Expansion of the genome projects to other invertebrates, such as molluscs and coelenterates, should provide new insights on the molecular correlates of building complex brains.

Enhancement of foot formation in Hydra by a novel epitheliopeptide, Hym-323

During the course of a systematic screening of peptide signaling molecules in Hydra magnipapillata, a novel peptide, Hym-323, which enhances foot regeneration was identified. The peptide is 16 amino acids long, and is encoded in the precursor protein as a single copy. Northern blot analysis, in situ hybridization analysis and immunohistochemistry showed that it was expressed in both ectodermal and endodermal epithelial cells throughout the body, except for the basal disk and the head region. The peptide enhanced foot regeneration by acting on epithelial cells. Lateral transplantation experiments indicated that the foot activation potential was increased in the peptide-treated tissue. These results suggest that Hym-323 is a peptide involved in a foot-patterning process in Hydra.

Role of epithelial cells and programmed cell death in Hydra spermatogenesis

Spermatogenesis in higher animals is a tightly regulated process, in which survival and death of sperm precursor cells depends on the presence of somatic cells in gonads. In the basal metazoan Hydra spermatogenesis takes place in anatomically simple testes and in the absence of accessory structures. Hydra sperm precursors are derived from interstitial stem cells. Here we show that large numbers of sperm precursors in testes of Hydra vulgaris undergo programmed cell death (apoptosis) and that ectodermal epithelial cells phagocytose the apoptotic sperm precursors. This is surprising since so far no evidence has been reported that epithelial cells are directly involved in germ cell differentiation in Hydra. We propose that, similar to Sertoli cells in mammals, in Hydra epithelial cells support and perhaps even control spermatogenesis.

Conserved and divergent genes in apex and axis development of cnidarians

Despite their radial organization and their sister group position in the life tree, cnidarian species express during morphogenesis a large number of genes that are related to bilaterian developmental genes. Among those, homologs to forkhead, emx, aristaless, goosecoid, brachyury, wnt and nanos genes are regulated during apical patterning in cnidarians, suggesting that key components of early organizer activity were conserved across evolution and recruited for either anterior, axial, or dorso-ventral patterning in bilaterians. In contrast, the expression patterns of the cnidarian Hox-related genes suggest that the apical-basal axis of the cnidarian polyp and the anterior-posterior axis of bilaterians do not differentiate following homologous processes.

The novel peptide HEADY specifies apical fate in a simple radially symmetric metazoan

One of the first steps in animal development is axis formation, during which an uneven distribution of signals and/or transcription factors results in the establishment of polarity in the embryo. Hydra, one of the simplest metazoan animals, shows characteristics of a permanent embryo. Even adult polyps have a striking capacity to regenerate, suggesting that molecular mechanisms underlying de novo pattern formation are permanently active and self regulatory. Here we show that HEADY, a short, amidated peptide, plays a central role in the specification of apical fate in this simple metazoan. The HEADY gene, whose transcripts accumulate at the apical organizing center, is required for specification of apical fate, as disruption of HEADY function by dsRNA mediated interference (RNAi) results in severe defects in head formation. Conversely, an instructive role of HEADY in head specification is demonstrated by the application of synthetic HEADY peptide, which induces formation of secondary axes with head morphology. Thus, the HEADY peptide acts as developmental switch to pattern the apical-basal axis of Hydra, providing a first insight into how initial asymmetry is specified in lower metazoan animals.

Genomic organization and splicing variants of a peptidylglycine alpha-hydroxylating monooxygenase from sea anemones

Cnidarians are primitive animals that use neuropeptides as their transmitters. All the numerous cnidarian neuropeptides isolated, so far, have a carboxy-terminal amide group that is essential for their actions. This strongly suggests that alpha-amidating enzymes are essential for the functioning of primitive nervous systems. In mammals, peptide amidation is catalyzed by two enzymes, peptidylglycine alpha-hydroxylating monooxygenase (PHM) and peptidyl-alpha-hydroxyglycine alpha-amidating lyase (PAL) that act sequentially. These two activities are contained within one bifunctional enzyme, peptidylglycine alpha-amidating monooxygenase (PAM), which is coded for by a single gene. In a previous paper (F. Hauser et al., Biochem. Biophys. Res. Commun. 241, 509-512, 1997) we have cloned the first known cnidarian PHM from the sea anemone Calliactis parasitica. In the present paper we have determined the structure of its gene (CP1). CP1 is >12 kb in size and contains 15 exons and 14 introns. The last coding exon (exon 15) contains a stop codon, leaving no room for PAL and, thereby, for a bifunctional PAM enzyme as in mammals. Furthermore, we found a CP1 splicing variant (CP1-B) that contains exon-9 instead of exon-8, which was present in the previously characterized PHM cDNA (CP1-A). CP1-A and -B have 97% amino acid sequence identity, whereas both splicing variants have around 42% sequence identity with the PHM part of rat PAM. Essential amino acid residues for the catalytic activity and the 3D structure of PHM are conserved between CP1-A, -B and the PHM part of rat PAM. Furthermore, eight introns in CP1 occur in the same positions and have the same intron phasing as eight introns in the rat PAM gene, showing that the sea anemone PHM is not only structurally, but also evolutionarily related to the PHM part of rat PAM.