Antho-RFamide-containing neurons in the primitive nervous system of the anthozoan Renilla koellikeri

Ultrastructural and immunocytochemical evidence of a colonial nervous system in hydroids

Background

As the sister group to all Bilateria, representatives of the phylum Cnidaria (sea anemones, corals, jellyfishes, and hydroids) possess a recognizable and well-developed nervous system and have attracted considerable attention over the years from neurobiologists and evo-devo researchers. Despite a long history of nervous system investigation in Cnidaria, most studies have been performed on unitary organisms. However, the majority of cnidarians are colonial (modular) organisms with unique and specific features of development and function. Nevertheless, data on the nervous system in colonial cnidarians are scarce. Within hydrozoans (Hydrozoa and Cnidaria), a structurally "simple" nervous system has been described for Hydra and zooids of several colonial species. A more complex organization of the nervous system, closely related to the animals' motile mode of life, has been shown for the medusa stage and a few siphonophores. Direct evidence of a colonial nervous system interconnecting zooids of a hydrozoan colony has been obtained only for two species, while it has been stated that in other studied species, the coenosarc lacks nerves.

Methods

In the present study, the presence of a nervous system in the coenosarc of three species of colonial hydroids - the athecate Clava multicornis, and thecate Dynamena pumila and Obelia longissima - was studied based on immunocytochemical and ultrastructural investigations.

Results

Confocal scanning laser microscopy revealed a loose system composed of delicate, mostly bipolar, neurons visualized using a combination of anti-tyrosinated and anti-acetylated a-tubulin antibodies, as well as anti-RF-amide antibodies. Only ganglion nerve cells were observed. The neurites were found in the growing stolon tips close to the tip apex. Ultrastructural data confirmed the presence of neurons in the coenosarc epidermis of all the studied species. In the coenosarc, the neurons and their processes were found to settle on the mesoglea, and the muscle processes were found to overlay the nerve cells. Some of the neurites were found to run within the mesoglea.

Discussion

Based on the findings, the possible role of the colonial nervous system in sessile hydroids is discussed.

Review: The evolution of peptidergic signaling in Cnidaria and Placozoa, including a comparison with Bilateria

Bilateria have bilateral symmetry and are subdivided into Deuterostomia (animals like vertebrates) and Protostomia (animals like insects and mollusks). Neuropeptides occur in both Proto- and Deuterostomia and they are frequently structurally related across these two lineages. For example, peptides belonging to the oxytocin/vasopressin family exist in both clades. The same is true for the G protein-coupled receptors (GPCRs) of these peptides. These observations suggest that these neuropeptides and their GPCRs were already present in the common ancestor of Proto- and Deuterostomia, which lived about 700 million years ago (MYA). Furthermore, neuropeptides and their GPCRs occur in two early-branching phyla that diverged before the emergence of Bilateria: Cnidaria (animals like corals and sea anemones), and Placozoa (small disk-like animals, feeding on algae). The sequences of these neuropeptides and their GPCRs, however, are not closely related to those from Bilateria. In addition, cnidarian neuropeptides and their receptors are not closely related to those from Placozoa. We propose that the divergence times between Cnidaria, Placozoa, and Bilateria might be too long for recognizing sequence identities. Leucine-rich repeats-containing GPCRs (LGRs) are a special class of GPCRs that are characterized by a long N-terminus containing 10-20 leucine-rich domains, which are used for ligand binding. Among the ligands for LGRs are dimeric glycoprotein hormones, and insulin-like peptides, such as relaxin. LGRs have been found not only in Proto- and Deuterostomia, but also in early emerging phyla, such as Cnidaria and Placozoa. Humans have eight LGRs. In our current review, we have revisited the annotations of LGRs from the sea anemone Nematostella vectensis and the placozoan Trichoplax adhaerens. We identified 13 sea anemone LGRs and no less than 46 LGRs from T. adhaerens. All eight human LGRs appear to have orthologues in sea anemones and placozoans. LGRs and their ligands, therefore, have a long evolutionary history, going back to the common ancestor of Cnidaria and Placozoa.

An evolutionary genomics view on neuropeptide genes in Hydrozoa and Endocnidozoa (Myxozoa)

BACKGROUND

The animal phylum Cnidaria consists of six classes or subphyla: Hydrozoa, Scyphozoa, Cubozoa, Staurozoa, Anthozoa, and Endocnidozoa. Cnidarians have an early evolutionary origin, diverging before the emergence of the Bilateria. Extant members from this phylum, therefore, are important resources for understanding the evolution of the nervous system. Cnidarian nervous systems are strongly peptidergic. Using genomics, we have recently shown that three neuropeptide families (the X1PRX2amides, GRFamides, and GLWamides) are wide-spread in four (Scyphozoa, Cubozoa, Staurozoa, Anthozoa) out of six cnidarian classes or subphyla, suggesting that these three neuropeptide families emerged in the common cnidarian ancestor. In the current paper, we analyze the remaining cnidarian class, Hydrozoa, and the subphylum Endocnidozoa, to make firm conclusions about the evolution of neuropeptide genes in Cnidaria.

RESULTS

We analyzed sixteen hydrozoan species with a sequenced genome or transcriptome, using a recently developed software program for discovering neuropeptide genes. These species belonged to various hydrozoan subclasses and orders, among them the laboratory models Hydra, Hydractinia, and Clytia. We found that each species contained three to five neuropeptide families. A common feature for all hydrozoans was that they contained genes coding for (i) X1PRX2amide peptides, (ii) GRFamide peptides, and (iii) GLWamide peptides. These results support our previous conclusions that these three neuropeptide families evolved early in evolution. In addition to these three neuropeptide families, hydrozoans expressed up to two other neuropeptide gene families, which, however, were only occurring in certain animal groups. Endocnidozoa (Myxozoa) are microscopically small endoparasites, which are strongly reduced. For long, it was unknown to which phylum these parasites belonged, but recently they have been associated with cnidarians. We analyzed nine endocnidozoan species and found that two of them (Polypodium hydriforme and Buddenbrockia plumatellae) expressed neuropeptide genes. These genes coded for neuropeptides belonging to the GRFamide and GLWamide families with structures closely resembling them from hydrozoans.

CONCLUSIONS

We found X1PRX2amide, GRFamide, and GLWamide peptides in all species belonging to the Hydrozoa, confirming that these peptides originated in the common cnidarian ancestor. In addition, we discovered GRFamide and GLWamide peptide genes in some members of the Endocnidozoa, thereby linking these parasites to Hydrozoa.

De novo transcriptome assembly of the cubomedusa Tripedalia cystophora, including the analysis of a set of genes involved in peptidergic neurotransmission

BACKGROUND

The phyla Cnidaria, Placozoa, Ctenophora, and Porifera emerged before the split of proto- and deuterostome animals, about 600 million years ago. These early metazoans are interesting, because they can give us important information on the evolution of various tissues and organs, such as eyes and the nervous system. Generally, cnidarians have simple nervous systems, which use neuropeptides for their neurotransmission, but some cnidarian medusae belonging to the class Cubozoa (box jellyfishes) have advanced image-forming eyes, probably associated with a complex innervation. Here, we describe a new transcriptome database from the cubomedusa Tripedalia cystophora.

RESULTS

Based on the combined use of the Illumina and PacBio sequencing technologies, we produced a highly contiguous transcriptome database from T. cystophora. We then developed a software program to discover neuropeptide preprohormones in this database. This script enabled us to annotate seven novel T. cystophora neuropeptide preprohormone cDNAs: One coding for 19 copies of a peptide with the structure pQWLRGRFamide; one coding for six copies of a different RFamide peptide; one coding for six copies of pQPPGVWamide; one coding for eight different neuropeptide copies with the C-terminal LWamide sequence; one coding for thirteen copies of a peptide with the RPRAamide C-terminus; one coding for four copies of a peptide with the C-terminal GRYamide sequence; and one coding for seven copies of a cyclic peptide, of which the most frequent one has the sequence CTGQMCWFRamide. We could also identify orthologs of these seven preprohormones in the cubozoans Alatina alata, Carybdea xaymacana, Chironex fleckeri, and Chiropsalmus quadrumanus. Furthermore, using TBLASTN screening, we could annotate four bursicon-like glycoprotein hormone subunits, five opsins, and 52 other family-A G protein-coupled receptors (GPCRs), which also included two leucine-rich repeats containing G protein-coupled receptors (LGRs) in T. cystophora. The two LGRs are potential receptors for the glycoprotein hormones, while the other GPCRs are candidate receptors for the above-mentioned neuropeptides.

CONCLUSIONS

By combining Illumina and PacBio sequencing technologies, we have produced a new high-quality de novo transcriptome assembly from T. cystophora that should be a valuable resource for identifying the neuronal components that are involved in vision and other behaviors in cubomedusae.

Global Neuropeptide Annotations From the Genomes and Transcriptomes of Cubozoa, Scyphozoa, Staurozoa (Cnidaria: Medusozoa), and Octocorallia (Cnidaria: Anthozoa)

During animal evolution, ancestral Cnidaria and Bilateria diverged more than 600 million years ago. The nervous systems of extant cnidarians are strongly peptidergic. Neuropeptides have been isolated and sequenced from a few model cnidarians, but a global investigation of the presence of neuropeptides in all cnidarian classes has been lacking. Here, we have used a recently developed software program to annotate neuropeptides in the publicly available genomes and transcriptomes from members of the classes Cubozoa, Scyphozoa, and Staurozoa (which all belong to the subphylum Medusozoa) and contrasted these results with neuropeptides present in the subclass Octocorallia (belonging to the class Anthozoa). We found three to six neuropeptide preprohormone genes in members of the above-mentioned cnidarian classes or subclasses, each coding for several (up to thirty-two) similar or identical neuropeptide copies. Two of these neuropeptide preprohormone genes are present in all cnidarian classes/subclasses investigated, so they are good candidates for being among the first neuropeptide genes evolved in cnidarians. One of these primordial neuropeptide genes codes for neuropeptides having the C-terminal sequence GRFamide (pQGRFamide in Octocorallia; pQWLRGRFamide in Cubozoa and Scyphozoa; pQFLRGRFamide in Staurozoa). The other primordial neuropeptide gene codes for peptides having RPRSamide or closely resembling amino acid sequences. In addition to these two primordial neuropeptide sequences, cnidarians have their own class- or subclass-specific neuropeptides, which probably evolved to serve class/subclass-specific needs. When we carried out phylogenetic tree analyses of the GRFamide or RPRSamide preprohormones from cubozoans, scyphozoans, staurozoans, and octocorallia, we found that their phylogenetic relationships perfectly agreed with current models of the phylogeny of the studied cnidarian classes and subclasses. These results support the early origins of the GRFamide and RPRSamide preprohormone genes.

The neuroendocrine immunomodulatory axis-like pathway mediated by circulating haemocytes in pacific oyster Crassostrea gigas

The neuroendocrine-immune (NEI) regulatory network is a complex system, which plays an indispensable role in the immunity of host. In this study, a neuroendocrine immunomodulatory axis (NIA)-like pathway mediated by the nervous system and haemocytes was characterized in the oyster Crassostrea gigas. Once invaded pathogen was recognized by the host, the nervous system would temporally release neurotransmitters to modulate the immune response. Instead of acting passively, oyster haemocytes were able to mediate neuronal immunomodulation promptly by controlling the expression of specific neurotransmitter receptors on cell surface and modulating their binding sensitivities, thus regulating intracellular concentration of Ca²⁺. This neural immunomodulation mediated by the nervous system and haemocytes could influence cellular immunity in oyster by affecting mRNA expression level of TNF genes, and humoral immunity by affecting the activities of key immune-related enzymes. In summary, though simple in structure, the ‘nervous-haemocyte’ NIA-like pathway regulates both cellular and humoral immunity in oyster, meaning a world to the effective immune regulation of the NEI network.

Muscle and nerve net organization in stalked jellyfish (Medusozoa: Staurozoa)

Staurozoan cnidarians display an unusual combination of polyp and medusa characteristics and their morphology may be informative about the evolutionary origin of medusae. We studied neuromuscular morphology of two staurozoans, Haliclystus 'sanjuanensis' and Manania handi, using whole mount immunohistochemistry with antibodies against FMRFamide and α-tubulin to label neurons and phalloidin to label muscles. All muscles appeared to lack striations. Longitudinal interradial muscles are probable homologues of stalk muscles in scyphopolyps, but in adult staurozoans they are elaborated to inwardly flex marginal lobes of the calyx during prey capture; these muscles are pennate in M. handi. Manubrial perradial muscles, like the manubrium itself, are an innovation shared with pelagic medusae and manubrial interradial muscles are shared with scyphozoan ephyra. Marginal muscles of M. handi displayed occasional synchronous contraction reminiscent of a medusa swim pulse, but contractions were not repetitive. The nerve net in both species showed regional variation in density and orientation of neurons. Some areas labeled predominantly by α-tubulin antibodies (exumbrellar epidermis), other areas labeled exclusively by FMRFamide antibodies (dense plexus of neurites surrounding the base of secondary tentacles, neuronal concentration at the base of transformed primary tentacles; gastrodermal nerve net), but most areas showed a mix of neurons labeled by these two antibodies and frequent co-labeling of neurons. Transformed primary tentacles had a concentration of FMRFamide-immunoreactive neurons at their base that was associated with a pigment spot in M. handi; this is consistent with their homology with rhopalia of medusae, which are also derived from primary tentacles. The muscular system of these staurozoans embodies characteristics of both scyphopolyps and pelagic medusae. However, their nerve net is more polyp-like, although marginal concentrations of the net associated with primary and secondary tentacles may facilitate the richer behavioral repertoire of staurozoans relative to polyps of other medusozoans. J. Morphol. 278:29-49, 2017. ©© 2016 Wiley Periodicals,Inc.

Fast neurotransmission related genes are expressed in non nervous endoderm in the sea anemone Nematostella vectensis

Cnidarian nervous systems utilize chemical transmission to transfer signals through synapses and neurons. To date, ample evidence has been accumulated for the participation of neuropeptides, primarily RFamides, in neurotransmission. Yet, it is still not clear if this is the case for the classical fast neurotransmitters such as GABA, Glutamate, Acetylcholine and Monoamines. A large repertoire of cnidarian Fast Neurotransmitter related Genes (FNGs) has been recently identified in the genome of the sea anemone, Nematostella vectensis. In order to test whether FNGs are localized in cnidarian neurons, we characterized the expression patterns of eight Nematostella genes that are closely or distantly related to human central and peripheral nervous systems genes, in adult Nematostella and compared them to the RFamide localization. Our results show common expression patterns for all tested genes, in a single endodermal cell layer. These expressions did not correspond with the RFamide expressing nerve cell network. Following these results we suggest that the tested Nematostella genes may not be directly involved in vertebrate-like fast neurotransmission.

Melatonin distribution reveals clues to its biological significance in basal metazoans

Although nearly ubiquitous in nature, the precise biological significance of endogenous melatonin is poorly understood in phylogenetically basal taxa. In the present work, we describe insights into the functional role of melatonin at the most “basal” level of metazoan evolution. Hitherto unknown morphological determinants of melatonin distribution were evaluated in Nematostella vectensis by detecting melatonin immunoreactivity and examining the spatial gene expression patterns of putative melatonin biosynthetic and receptor elements that are located at opposing ends of the melatonin signaling pathway. Immuno-melatonin profiling indicated an elaborate interaction with reproductive tissues, reinforcing previous conjectures of a melatonin-responsive component in anthozoan reproduction. In situ hybridization (ISH) to putative melatonin receptor elements highlighted the possibility that the bioregulatory effects of melatonin in anthozoan reproduction may be mediated by interactions with membrane receptors, as in higher vertebrates. Another intriguing finding of the present study pertains to the prevalence of melatonin in centralized nervous structures. This pattern may be of great significance given that it 1) identifies an ancestral association between melatonin and key neuronal components and 2) potentially implies that certain effects of melatonin in basal species may be spread widely by regionalized nerve centers.

Temporal and histological evaluation of melatonin patterns in a 'basal' metazoan

While recent advances suggest functional pleiotropy of melatonin in higher organisms, an understanding of the biological significance of this ancient molecule in early evolutionary groups is lacking. Here, endogenous melatonin production was identified for the first time in the sea anemone Actinia equina, a nonsymbiotic hexacorallian cnidarian. Day/night activity profiles of melatonin in this anemone indicated that melatonin levels oscillate with significant nocturnal peaks. However, dynamic changes in melatonin concentration did not persist under constant dark conditions and therefore were not circadian in nature. Thus, the oscillating pattern of melatonin in A. equina is presumed to be the result of alternative, simpler melatonin control mechanism that likely involves direct regulation by the daily photocycle. As nocturnal melatonin signals still potentially provide 'time-of-day' information and can illustrate the seasonally changing length of the biological night, we hypothesize that melatonin may be relevant to temporal coordination of timed processes also in anthozoans. Spatial patterns of melatonin distribution found in this study indicate abundant melatonin distribution in the endodermal filaments wrapped around gametes. This finding supports the possibility that one of the melatonin-responsive processes in this basal metazoan species may involve reproductive functions.

A review of FMRFamide- and RFamide-like peptides in metazoa

Neuropeptides are a diverse class of signalling molecules that are widely employed as neurotransmitters and neuromodulators in animals, both invertebrate and vertebrate. However, despite their fundamental importance to animal physiology and behaviour, they are much less well understood than the small molecule neurotransmitters. The neuropeptides are classified into families according to similarities in their peptide sequence; and on this basis, the FMRFamide and RFamide-like peptides, first discovered in molluscs, are an example of a family that is conserved throughout the animal phyla. In this review, the literature on these neuropeptides has been consolidated with a particular emphasis on allowing a comparison between data sets in phyla as diverse as coelenterates and mammals. The intention is that this focus on the structure and functional aspects of FMRFamide and RFamide-like neuropeptides will inform understanding of conserved principles and distinct properties of signalling across the animal phyla.

Prominent system of RFamide immunoreactive neurons in the rhopalia of box jellyfish (Cnidaria: Cubozoa)

The four visual sensory structures of a cubomedusa, the rhopalia, display a surprisingly elaborate organization by containing two lens eyes and four bilaterally paired pigment cup eyes. Peptides containing the peptide sequence Arg-Phe-NH2 (RFamide) occur in close association with visual structures of cnidarians, including the rhopalia and rhopalial stalk of cubomedusae, suggesting that RFamide functions as a neuronal marker for certain parts of the visual system of medusae. Using immunofluorescence we give a detailed description of the organization of the RFamide-immunoreactive (ir) nervous system in the rhopalia and rhopalial stalk of the cubomedusae Tripedalia cystophora and Carybdea marsupialis. The bilaterally symmetric RFamide-ir nervous system contains four cell groups and three morphologically different cell types. Neurites spread throughout the rhopalia and occur in close vicinity of the pigment cup eyes and the lower lens eye. Two commissures connect the two sides of the system and neurites of one rhopalial cell group extend into the rhopalial stalk. The RFamide-ir nervous system in the rhopalia of cubomedusae is more widespread and comprises more cells than earlier discerned. We suggest that the system might not only integrate visual input but also signals from other senses. One of the RFamide-ir cell groups is favorably situated to represent pacemaker neurons that set the swimming rhythm of the medusa.

Origins of neurogenesis, a cnidarian view

New perspectives on the origin of neurogenesis emerged with the identification of genes encoding post-synaptic proteins as well as many "neurogenic" regulators as the NK, Six, Pax, bHLH proteins in the Demosponge genome, a species that might differentiate sensory cells but no neurons. However, poriferans seem to miss some key regulators of the neurogenic circuitry as the Hox/paraHox and Otx-like gene families. Moreover as a general feature, many gene families encoding evolutionarily-conserved signaling proteins and transcription factors were submitted to a wave of gene duplication in the last common eumetazoan ancestor, after Porifera divergence. In contrast gene duplications in the last common bilaterian ancestor, Urbilateria, are limited, except for the bHLH Atonal-class. Hence Cnidaria share with Bilateria a large number of genetic tools. The expression and functional analyses currently available suggest a neurogenic function for numerous orthologs in developing or adult cnidarians where neurogenesis takes place continuously. As an example, in the Hydra polyp, the Clytia medusa and the Acropora coral, the Gsx/cnox2/Anthox-2 ParaHox gene likely supports neurogenesis. Also neurons and nematocytes (mechanosensory cells) share in hydrozoans a common stem cell and several regulatory genes indicating that they can be considered as sister cells. Performed in anthozoan and medusozoan species, these studies should tell us more about the way(s) evolution hazards achieved the transition from epithelial to neuronal cell fate, and about the robustness of the genetic circuitry that allowed neuromuscular transmission to arise and be maintained across evolution.

Immunohistochemical localization of a retinoic acid-like receptor in nerve cells of two colonial anthozoans (Cnidaria)

Retinoic acid is known to induce vertebrate stem cells to differentiate into a variety of cell types, including neurons. Although retinoic acid was reported to affect morphogenetic pattern specification in the hydrozoan Hydractinia (Müller, W.A., 1984. Retinoids and pattern formation in a hydroid. J. Embryol. Exp. Morph. 81, 253-271) and a retinoid RXR receptor was cloned in the jellyfish Tripedalia (Kostrouch, Z., Kostrouchova, M., Love, W., Jannini, E., Piatigorsky, J., Rall, J.E., 1998. Retinoic acid X receptor in the diploblast, Tripedalia cystophora. Proc. Natl. Acad. Sci. U.S.A. 95, 13442-13447), the cellular targets of retinoids were not investigated. We used Western immunoblotting and immunohistochemistry to investigate the presence and cellular distribution of a RXR-like receptor in the sea pansy Renilla koellikeri and in the staghorn coral Acropora millepora (Cnidaria, Anthozoa). Western blots revealed a 64 kDa protein from a sea pansy extract in a band that co-migrated with a RXR protein from the rat brain. Using antibodies raised against an epitope of human alpha RXR, we visualized putative ectodermal sensory cells in the polyp column of the adult sea pansy. Immunoreactivity was absent in staghorn coral larvae but present in the polyp column of adult colonies in the form of clusters of neuron-like cells in the basiectoderm near the ectoderm-mesoglea interface. These observations suggest that a RXR-like receptor is involved in epithelial nerve cell specification in adult anthozoans and that this role is conserved throughout evolution.

FMRFamide-like immunoreactivity in the sea-fan Eunicella cavolini (Cnidaria: Octocorallia)

The presence of FMRFamide-related peptides (FaRPs) was investigated, by immunohistochemical methods with a polyclonal FMRFamide antiserum, in the sea-fan Eunicella cavolini (Van Koch 1887), a representative of the cnidarians (octocorallians). The identification of FaRP-immunoreactive elements as neuronal cells and a nerve net was performed by double immunohistochemical methods with the monoclonal anti-beta-tubulin antibody. A strong and widely distributed FaRPs immunoreactivity was detected: FaRPs-immunoreactive nerve cells were observed among and underlying gastrodermal epithelial cells, epidermal cells lining tentacles, muscular septs and gonophores. A diffuse FaRPs-immunoreactive nerve net was also found between epithelia and mesoglea and in the stalk of the gonophore. These results improve our knowledge of the gorgonian nervous system and demonstrate that most of the immunoreactive cells belong to neural elements.

Spawning and gamete follicle rupture in the cnidarian Renilla koellikeri: effects of putative neurohormones

The neuroendocrine control of spawning (release of intact gamete follicles) and of the ensuing exfoliation (freeing of gametes by follicle epithelium rupture) was investigated in colonies of the sea pansy Renilla koellikeri, an octocorallian of the sea pen family. Polyps of male colonies produce substantially more sperm follicles than female colonies do egg follicles, and significantly more sperm follicles are expelled than egg follicles during the summer spawning season. Spawning is accompanied by strong peristaltic contractions across the colony. Serotonin, a positive modulator of peristalsis in the sea pansy, induced spawning of either sperm or egg follicles, increasing both the proportion of spawning colonies and the number of expelled gamete follicles per colony in a dose-dependent manner. The serotonin antagonist 1-(1)naphthylpiperazine greatly reduced both spontaneous and serotonin-induced spawning. Antho-RFamide, a neuropeptide found in ciliated neurons within follicle epithelia, induced the exfoliation of the follicle epithelium from spawned follicles. Exposure of follicles to light enhanced the potency of Antho-RFamide. The actin-binding toxin phalloidin substantially reduced the incidence of Antho-RFamide-induced exfoliation and phalloidin-FITC staining was localized in the muscle feet of follicle epithelial cells. These results provide the first experimental evidence of neuroendocrine functions involved in cnidarian spawning.