Control of foot differentiation in Hydra: Phylogenetic footprinting indicates interaction of head, bud and foot patterning systems

Homeodomain transcription factor CnNK-2 seems to play a major role in foot formation in Hydra. Recently, we reported in vitro evidence indicating that CnNK-2 has autoregulatory features and regulates expression of the morphogenetic peptide pedibin. We proposed that CnNK-2 and pedibin synergistically orchestrate foot differentiation processes. Here, we further analyzed the regulatory network controlling foot formation in Hydra. By phylogenetic footprinting we compared the CnNK-2 5'-flanking sequence from two closely related species, Hydra vulgaris and Hydra oligactis. Unexpectedly, we detected a highly conserved binding site for HNF-3beta, a vertebrate Forkhead transcription factor, in the CnNK-2 5'-flanking region. The Hydra HNF-3beta homolog budhead is predominantly expressed in the apical region of the body column and early during budding. Budhead is absent from tissue expressing CnNK-2 and thought to be involved in determining tissue for head differentiation. By electrophoretic mobility shift assays we demonstrate an in vitro interaction between recombinant budhead protein and the interspecific conserved HNF-3beta binding motif in the CnNK-2 5'-flanking region. Our results strengthen the view of CnNK-2 as an important regulator during foot patterning processes. Furtheron, they point to budhead as a candidate for a transcriptional regulator of CnNK-2 and to an interaction of foot and head patterning processes in Hydra on the molecular level.

SymbioticHydraexpress a plant-like peroxidase gene during oogenesis

Symbiotic associations accompanied by gene exchange between the symbionts form the phylogenetic origin of eukaryotic cells and, therefore, had significant impact on species diversity and evolutionary novelty. Among the phylogenetically oldest metazoan animals known to form symbiotic relationships are the Cnidaria. In the Cnidarian Hydra viridis, symbiotic algae of the genus Chlorella are located in endodermal epithelial cells and impact sexual differentiation. When screening for Hydra viridis genes that are differentially expressed during symbiosis, we found a gene, HvAPX1, coding for a plant-related ascorbate peroxidase. HvAPX1 is expressed exclusively during oogenesis and in contrast to all known ascorbate peroxidase genes in plants does not contain introns. No member of this gene family has previously been identified from a member of the animal kingdom. We discuss the origin of the HvAPX1 gene and propose that it may have been transferred horizontally following an endosymbiotic event early in evolution of the Hydra lineage as an RNA or cDNA intermediate.

Towards a molecular code for individuality in the absence of MHC: screening for individually variable genes in the urochordate Ciona intestinalis

Urochordates possess several well described allorecognitions systems, the molecular nature of those is not yet understood. A prerequisite for any self-/nonself discimination system is the presence of a group of highly variable molecules, which should vary between individuals. Using suppression subtractive hybridisation (SSH) we surveyed Ciona intestinalis for individually variable genes. Our search so far identified two genes, ciS7 and ciMETA2, which both display an unexpected high degree of intra- and interindividual variability, code for secreted proteins, and contain multiple domains suitable for protein-protein interactions. The possible role of these molecules in allorecognition is discussed.

Recognition strategies in the innate immune system of ancestral chordates

Many components of the innate immune system in vertebrates can be reliably traced to urochordates and successful strategies for the detection and elimination of pathogens are present at that level of animal evolution, but the issue of where and how the adaptive immune system emerged is still obscure. There is a paucity of evidence for a gradual transition from the innate immune system of invertebrates to the recombinatorial immune system of higher vertebrates. None of the classical elements of MHC based transplantation immunity (MHC, TCR) or humoral immunity (Ig) have been found in urochordates or Agnathans. Nevertheless there is abundant evidence for adaptive immune responses in the agnathans. This remarkable paradox raises a number of questions. How do these ancestral chordates discriminate between the constituents of the external world and the constituents of "self"? Are these strategies universal within the animal kingdom and among chordates, or are different strategies used by representatives of the different taxonomic groups? The current state of our knowledge indicates that the immune system of lower chordates is very different from that of cartilaginous fishes. Pure homology hunting for vertebrate-specific immuno-relevant molecules in invertebrates is therefore of limited value. A more promising approach may involve unbiased functional screening methods. To understand better the evolution of adaptive immune systems, more comparative data from jawless vertebrates (lamprey or hagfish) and a representative of Acrania (e.g. Amphioxus) are clearly needed.

Ancient signals: peptides and the interpretation of positional information in ancestral metazoans

Understanding the 'tool kit' that builds the most fundamental aspects of animal complexity requires data from the basal animals. Among the earliest diverging animal phyla are the Cnidaria which are the first in having a defined body plan including an axis, a nervous system and a tissue layer construction. Here I revise our understanding of patterning mechanism in cnidarians with special emphasis on the nature of positional signals in Hydra as perhaps the best studied model organism within this phylum. I show that (i) peptides play a major role as positional signals and in cell-cell communication; (ii) that intracellular signalling pathways in Hydra leading to activation of target genes are shared with all multicellular animals; (iii) that homeobox genes translate the positional signals; and (iv) that the signals are integrated by a complex genetic regulatory machinery that includes both novel cis regulatory elements as well as taxon specific target genes. On the basis of these results I present a model for the regulatory interactions required for axis formation in Hydra.

A Dickkopf - 3 -related gene is expressed in differentiating nematocytes in the basal metazoan Hydra

In vertebrate development the Dickkopf protein family carries out multiple functions and is represented by at least four different genes with distinct biological activities. In invertebrates such as Drosophila and Caenorhabditis, Dickkopf genes have so far not been identified. Here we describe the identification and characterization of a Dickkopf gene with a deduced amino acid sequence closely related to that of chicken Dkk-3 in the basal metazoan Hydra. HyDkk-3 appears to be the only Dickkopf gene in Hydra. The gene is expressed in the gastric region in nematocytes at a late differentiation stage. In silico searches of EST and genome databases indicated the absence of Dkk genes from the protostomes Drosophila and Caenorhabditis, whereas within the deuterostomes, a Dkk-3 gene could be identified in the genome of the urochordate Ciona intestinalis. The results indicate that at an early stage of evolution of multicellularity Dickkopf proteins have already played important roles as developmental signals. They also suggest that vertebrate Dkk-1, 2 and 4 may have originated from a common ancestor gene of Dkk-3.

Control of foot differentiation in Hydra: in vitro evidence that the NK-2 homeobox factor CnNK-2 autoregulates its own expression and uses pedibin as target gene

The foot of the simple metazoan Hydra is a highly dynamic body region of constant tissue movement, cell proliferation, determination and differentiation. Previously, two genes have been shown to participate in the development and differentiation of this body region: homeodomain factor CnNK-2 and signal peptide pedibin [Dev. Biol. 180 (1996) 473; Development 126 (1999) 517; Development 122 (1996) 1941; Mech. Dev. 106 (2001) 37]. CnNk-2 functions as transcriptional regulator and is responsive to changes in the positional value while the secreted peptide pedibin serves as "extrinsic" positional signal. Exposure of polyps to pedibin increases the spatial domain of CnNK-2 expression towards the gastric region, indicating that positional signals are integrated at the cis-regulatory region of CnNK-2. In the present study, to elucidate the molecular basis of the interaction of CnNK-2 and pedibin, we characterized the 5' regulatory regions of both genes. Within the CnNK-2 5' upstream region, electrophoretic mobility shift assays showed that putative NK-2 binding motifs are specifically bound by both nuclear protein from Hydra foot and by recombinant CnNK-2, suggesting that CnNK-2 might autoregulate its own expression. This is the first indication for an autoregulatory circuit in Hydra. In addition, we also identified NK-2 binding sites in the cis-regulatory region of the pedibin gene, indicating that this gene is one of the targets of the transcription factor CnNK-2. On the basis of these results, we present a model for the regulatory interactions required for patterning the basal end of the single axis in Hydra which postulates that CnNK-2 together with pedibin orchestrates foot specific differentiation.

Control of asymmetric cell divisions: will cnidarians provide an answer?

Cells in the basal metazoan phylum Cnidaria are characterized by remarkable plasticity in their differentiation capacity. The mechanism controlling asymmetric cell divisions is not understood in cnidarians or in any other animal group. PIWI proteins recently have been shown to be involved in maintaining the self-renewal capacity of stem cells in organisms as diverse as ciliates, flies, worms and mammals. Seipel et al.1 find that, in the cnidarian Podocoryne carnea, the Piwi homolog Cniwi is transcriptionally upregulated when the polyp generates buds, which will develop into medusae. Since transdifferentiation of striated muscle cells to smooth muscle cells also activated Cniwi expression, Cniwi appears to play a crucial role in differentiation events. The discovery should facilitate elucidation of the poorly understood factors that control asymmetric cell divisions at the beginning of animal evolution.

Expression of developmental genes during early embryogenesis of Hydra

Hydra is a classical model to study key features of embryogenesis such as axial patterning and stem cell differentiation. In contrast to other organisms where these mechanisms are active only during embryonic development, in Hydra they can be studied in adults. The underlying assumption is that the machinery governing adult patterning mimics regulatory mechanisms which are also active during early embryogenesis. Whether, however, Hydra embryogenesis is governed by the same mechanisms which are controlling adult patterning, remains to be shown. In this paper, in precisely staged Hydra embryos, we examined the expression pattern of 15 regulatory genes shown previously to play a role in adult patterning and cell differentiation. RT-PCR revealed that most of the genes examined were expressed in rather late embryonic stages. In situ hybridization, nuclear run-on experiments, and staining of nucleolar organizer region-associated proteins indicated that genes expressed in early embryos are transcribed in the engulfed "nurse cells" (endocytes). This is the first direct evidence that endocytes in Hydra not only provide nutrients to the developing oocyte but also produce maternal factors critical for embryogenesis. Our findings are an initial step towards understanding the molecular machinery controlling embryogenesis of a key group of basal metazoans and raise the possibility that in Hydra there are differences in the mechanisms controlling embryogenesis and adult patterning.

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.

Urochordates and the origin of natural killer cells: identification of a CD94/NKR-P1-related receptor in blood cells of Botryllus

Transplantation immunity based on the recognition of MHC molecules is well described in vertebrates. Vertebrates, however, do not undergo transplantation reaction naturally. The phylogenetically closest group in which transplantation reactions can occur is the Urochordata. Therefore, these animals occupy a key position for understanding the evolution of the vertebrate immune system. When screening for genes differentially expressed during allorecognition in Botryllus schlosseri, we isolated a gene coding for a type II transmembrane protein with a C-type lectin-binding domain and close similarity to vertebrates CD94 and NKR-P1. Here we show that the gene, BsCD94-1, is differentially regulated during allorecognition and that a subpopulation of blood cells carries the corresponding receptor on its cell surface. Southern blot analysis with DNA from individual colonies and intronless BsCD94-1 probe reveal variation between individuals at the genomic level. CD94 in vertebrates is one of the markers for natural killer cells and binds to MHC class I molecules. Natural killer cells play a major role in recognition and elimination of allogeneic cells. Their evolutionary origin, however, remained unknown. The results presented here indicate that the elaboration of the vertebrate immune system may have its roots in an ancestral population of cells in the urochordate blood.

The Hydra viridis / Chlorella symbiosis. Growth and sexual differentiation in polyps without symbionts

To investigate interactions between the basal metazoan Hydra viridis and its symbiotic Chlorella algae, we generated aposymbiotic hydra lacking algae and compared them to symbiotic ones with regard to growth and sexual differentiation. Under standard feeding conditions aposymbiotic polyps proliferated similarly to symbiotic polyps. Under moderate and low feeding conditions asexual growth was reduced in polyps lacking algae, indicating that the symbionts supply nutrients to their hosts. In addition, the Chlorella symbionts had a strong influence on the sexual reproduction of Hydra viridis: in most cases female gonads were produced only when symbiotic algae were present. Spermatogenesis proceeded similarly in symbiotic and aposymbiotic polyps. Since during oogenesis symbionts are actively transferred from endodermal epithelial cells to the ectodermal oocytes, this oogenesis promoting role could indicate that the symbionts are critically involved in the control of sexual differentiation in green hydra.

Epithelial interactions in Hydra: apoptosis in interspecies grafts is induced by detachment from the extracellular matrix

Apoptosis plays an important role in immunity and is widely used to eliminate foreign or infected cells. Cnidaria are the most basal eumetazoans and have no specialised immune cells, but some colonial cnidarians possess a genetic system to discriminate between self and non-self. By grafting epithelia of different species we have previously shown that the freshwater polyp Hydra eliminates non-self cells by phagocytosis. Here we have investigated whether apoptosis is involved in the histocompatibility reactions. We studied epithelial interactions between Hydra vulgaris and Hydra oligactis and show that a large number of apoptotic cells accumulate in the contact region of interspecies grafts. Histological analysis of the graft site revealed that displacement of the endodermal layer of Hydra vulgaris by endoderm from Hydra oligactis coincided with impaired cell—cell and cell—matrix contacts. We therefore suggest that in interspecies grafts, apoptosis is induced by the detachment of epithelial cells from the extracellular matrix(anoikis) and not by a discriminative allorecognition system.

In vivo electroporation for genetic manipulations of whole Hydra polyps

In vivo electroporation is used to study gene regulation and gene function in the freshwater polyp Hydra. Although this approach has been used successfully by several investigators, efficacy and handling continue to present a problem. Here we show technical aspects of in vivo electroporation for introducing fluorescent dyes, plasmid DNA and double stranded RNA into Hydra polyps. We describe the fundamentals of the electroporation delivery system, discuss recent studies where this approach has been used successfully, compare it to alternative transfection methods such as lipofection, and identify future directions.