Isolation of two novel neuropeptides from sea anemones: the unusual, biologically active L-3-phenyllactyl-Tyr-Arg-Ile-NH2 and its des-phenyllactyl fragment Tyr-Arg-Ile-NH2

A comparative genomics study of neuropeptide genes in the cnidarian subclasses Hexacorallia and Ceriantharia

Background

Nervous systems originated before the split of Proto- and Deuterostomia, more than 600 million years ago. Four animal phyla (Cnidaria, Placozoa, Ctenophora, Porifera) diverged before this split and studying these phyla could give us important information on the evolution of the nervous system. Here, we have annotated the neuropeptide preprohormone genes of twenty species belonging to the subclass Hexacorallia or Ceriantharia (Anthozoa: Cnidaria), using thirty-seven publicly accessible genome or transcriptome databases. Studying hexacorals is important, because they are versatile laboratory models for development (e.g., Nematostella vectensis) and symbiosis (e.g., Exaiptasia diaphana) and also are prominent reef-builders.

Results

We found that each hexacoral or ceriantharian species contains five to ten neuropeptide preprohormone genes. Many of these preprohormones contain multiple copies of immature neuropeptides, which can be up to 50 copies of identical or similar neuropeptide sequences. We also discovered preprohormones that only contained one neuropeptide sequence positioned directly after the signal sequence. Examples of them are neuropeptides that terminate with the sequence RWamide (the Antho-RWamides). Most neuropeptide sequences are N-terminally protected by pyroglutamyl (pQ) or one or more prolyl residues, while they are C-terminally protected by an amide group. Previously, we isolated and sequenced small neuropeptides from hexacorals that were N-terminally protected by an unusual L-3-phenyllactyl group. In our current analysis, we found that these N-phenyllactyl-peptides are derived from N-phenylalanyl-peptides located directly after the signal sequence of the preprohormone. The N-phenyllactyl- peptides appear to be confined to the hexacorallian order Actiniaria and do not occur in other cnidarians. On the other hand, (1) the neuropeptide Antho-RFamide (pQGRFamide); (2) peptides with the C-terminal sequence GLWamide; and (3) tetrapeptides with the X₁PRX₂amide consensus sequence (most frequently GPRGamide) are ubiquitous in Hexacorallia.

Conclusions

We found GRFamide, GLWamide, and X₁PRX₂amide peptides in all tested Hexacorallia. Previously, we discovered these three neuropeptide classes also in Cubozoa, Scyphozoa, and Staurozoa, indicating that these neuropeptides originated in the common cnidarian ancestor and are evolutionarily ancient. In addition to these ubiquitous neuropeptides, other neuropeptides appear to be confined to specific cnidarian orders or subclasses.

Chemical transmission in the sea anemone Nematostella vectensis: A genomic perspective

The sequencing of the starlet sea anemone (Nematostella vectensis) genome provides opportunities to investigate the function and evolution of genes associated with chemical neurotransmission and hormonal signaling. This is of particular interest because sea anemones are anthozoans, the phylogenetically basal cnidarians least changed from the common ancestors of cnidarians and bilaterian animals, and because cnidarians are considered the most basal metazoans possessing a nervous system. This analysis of the genome has yielded 20 orthologues of enzymes and nicotinic receptors associated with cholinergic function, an even larger number of genes encoding enzymes, receptors and transporters for glutamatergic (28) and GABAergic (34) transmission, and two orthologues of purinergic receptors. Numerous genes encoding enzymes (14), receptors (60) and transporters (5) for aminergic transmission were identified, along with four adenosine-like receptors and one nitric oxide synthase. Diverse neuropeptide and hormone families are also represented, mostly with genes encoding prepropeptides and receptors related to varying closeness to RFamide (17) and tachykinin (14), but also galanin (8), gonadotropin-releasing hormones and vasopressin/oxytocin (5), melanocortins (11), insulin-like peptides (5), glycoprotein hormones (7), and uniquely cnidarian peptide families (44). Surprisingly, no muscarinic acetylcholine receptors were identified and a large number of melatonin-related, but not serotonin, orthologues were found. Phylogenetic tree construction and inspection of multiple sequence alignments reveal how evolutionarily and functionally distant chemical transmitter-related proteins are from those of higher metazoans.

The construction of a bioactive peptide database in Metazoa

Bioactive peptides play critical roles in regulating most biological processes in animals, and have considerable biological, medical and industrial importance. A number of peptides have been discovered usually based on their biological activities in vitro or based on their sequence similarities in silico. Through searches in Swiss-Prot and Trembl protein databases using BLAST alignment tools and other in silico methods, all currently known bioactive peptides and their precursor proteins are extracted. In addition, 132 recently discovered putative peptide genes in Drosophila as well as their orthologs in other species are collected. In total, 20 027 bioactive peptides from 19 438 precursor proteins covering 2820 metazoan species are retained, and they, respectively, make up a peptide and a peptide precursor database. The peptides and peptide precursor proteins are further classified into 373 families, 178 of which are represented by Prosite Pfam or Smart motifs, or by typical peptide motifs that have been constructed recently. The remaining 195 families are novel peptide families. The motifs characterizing the 178 peptide families are saved into a peptide motif database. The peptide, peptide precursor and peptide motif databases (version 1.0) are the most complete peptide, precursor and peptide motif collection in Metazoa so far. They are available on the WWW at http://www.peptides.be/.

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.

Peptides in the nervous systems of cnidarians: structure, function, and biosynthesis

Cnidarians are the lowest animal group having a nervous system and it was probably within this phylum or in a related ancestor group that nervous systems first evolved. The primitive nervous systems of cnidarians are strongly peptidergic. From a single sea anemone species, Anthopleura elegantissima, 17 different neuropeptides have been isolated so far, and we expect that many more neuropeptides (more than 30) must be present. All peptides are localized in neurons of cnidarians and we have demonstrated the presence of some of the peptides in neurosecretory dense-cored vesicles. Most neuropeptides have an excitatory or inhibitory action on whole cnidarians, muscle preparations, and isolated muscle cells, suggesting that these peptides are neurotransmitters or neuromodulators. One neuropeptide induces metamorphosis in planula larvae to become a polyp. This shows that cnidarian neuropeptides also are involved in developmental processes, such as cell differentiation and pattern formation. We have cloned the preprohormones for most of the cnidarian neuropeptides. These preprohormones have a high copy number of the immature neuropeptide sequence, which can be up to 37 neuropeptide copies per precursor molecule. In addition to well-known, "classical" processing enzymes, novel prohormone processing enzymes must be present in cnidarian neurons. These include a processing enzyme hydrolyzing at the C-terminal sides of acidic (Asp and Glu) residues and a dipeptidyl aminopeptidase digesting at the C-terminal sides of N-terminally located X-Pro and X-Ala sequences. All this shows that the primitive nervous systems of cnidarians are already quite complex, and that neuropeptides play a central role in the physiology of these animals.

The nervous systems of cnidarians

Cnidarians have simple nervous systems and it was probably within this group or a closely-related ancestor that nervous systems first evolved. The basic plan of the cnidarian nervous system is that of a nerve net which, at some locations, has condensed to form nerve plexuses, or circular or longitudinal nerve tracts which may be syncytia. At the ultrastructural level, many cnidarian neurons have the combined characteristics of sensory, motor, inter- and neurosecretory neurons and thus appear to be multifunctional. We propose that these multifunctional neurons resemble the ancestors of the more specialized neurons that we find in higher animals today. The primitive nervous system of cnidarians is strongly peptidergic: from a single sea anemone species Anthopleura elegantissima, we have now isolated 16 different novel neuropeptides. These peptides are biologically active and cause inhibitions or contractions in muscle preparations or isolated muscle cells from sea anemones. The various peptides are located in at least six distinct sets of neurons showing that sea anemone neurons have already specialized with respect to their peptide content. Using immuno-electronmicroscopy, we have found that the peptides are located in neuronal dense-cored vesicles associated with both synaptic and non-synaptic release sites. All these data indicate that evolutionarily "old" nervous systems use peptides as transmitters. We have also investigated the biosynthesis of the cnidarian neuropeptides. These neuropeptides are made as large precursor proteins which contain multiple (up to 36) copies of immature neuropeptides. Thus, the biosynthesis of neuropeptides in cnidarians is very efficient and comparable to that of higher invertebrates, such as molluscs and insects, and vertebrates.

The expansion behaviour of sea anemones may be coordinated by two inhibitory neuropeptides, Antho-KAamide and Antho-RIamide

Antho-KAamide (L-3-phenyllactyl-Phe-Lys-Ala-NH2) and Antho-RIamide (L-3-phenyllactyl-Tyr-Arg-Ile-NH2) are novel neuropeptides isolated from the sea anemone Anthopleura elegantissima. They both inhibited spontaneous contractions of isolated muscle preparations from a wide variety of anemone species (threshold around 10(-7) M). Their actions were universal in that they inhibited every muscle preparation tested, regardless of whether the muscle group was located in the ectoderm or endoderm, or was oriented in a circular or longitudinal direction. Injection of Antho-KAamide or Antho-RIamide into the coelenteron of intact sea anemones resulted in a marked expansion of the animals. Similar shape changes followed feeding or exposure to soluble food extracts. Therefore, we hypothesize that nerve cells that release Antho-KAamide and Antho-RIamide are involved in the expansion phase of feeding behaviour in sea anemones.

Isolation of Leu-Pro-Pro-Gly-Pro-Leu-Pro-Arg-Pro-NH2 (Antho-RPamide), an N-terminally protected, biologically active neuropeptide from sea anemones

Using a radioimmunoassay against the C-terminal sequence Arg-Pro-NH2 (RPamide), we have isolated the peptide Leu-Pro-Pro-Gly-Pro-Leu-Pro-Arg-Pro-NH2 (Antho-RPamide) from an extract of the sea anemone Anthopleura elegantissima. Antho-RPamide is located in neurons of sea anemones. Application of low concentrations of Antho-RPamide to tentacle preparations of sea anemones strongly increased the frequency and duration of spontaneous contractions, suggesting that this peptide is involved in neurotransmission. Antho-RPamide has a free N-terminus, yet its X-Pro-Pro sequence makes it relatively resistant to degradation by nonspecific aminopeptidases. Thus, we have discovered another strategy by which sea anemones protect the N-termini of their bioactive neuropeptides.