Metamorphosin A and Related Compounds: A Novel Family of Neuropeptides with Morphogenic Activitya

The Nereid on the rise: Platynereis as a model system

The Nereid Platynereis dumerilii (Audouin and Milne Edwards (Annales des Sciences Naturelles 1:195-269, 1833) is a marine annelid that belongs to the Nereididae, a family of errant polychaete worms. The Nereid shows a pelago-benthic life cycle: as a general characteristic for the superphylum of Lophotrochozoa/Spiralia, it has spirally cleaving embryos developing into swimming trochophore larvae. The larvae then metamorphose into benthic worms living in self-spun tubes on macroalgae. Platynereis is used as a model for genetics, regeneration, reproduction biology, development, evolution, chronobiology, neurobiology, ecology, ecotoxicology, and most recently also for connectomics and single-cell genomics. Research on the Nereid started with studies on eye development and spiralian embryogenesis in the nineteenth and early twentieth centuries. Transitioning into the molecular era, Platynereis research focused on posterior growth and regeneration, neuroendocrinology, circadian and lunar cycles, fertilization, and oocyte maturation. Other work covered segmentation, photoreceptors and other sensory cells, nephridia, and population dynamics. Most recently, the unique advantages of the Nereid young worm for whole-body volume electron microscopy and single-cell sequencing became apparent, enabling the tracing of all neurons in its rope-ladder-like central nervous system, and the construction of multimodal cellular atlases. Here, we provide an overview of current topics and methodologies for P. dumerilii, with the aim of stimulating further interest into our unique model and expanding the active and vibrant Platynereis community.

Comparative Aspects of Structure and Function of Cnidarian Neuropeptides

Cnidarians are early-branching animals in the eukaryotic tree of life. The phylum Cnidaria are divided into five classes: Scyphozoa (true jellyfish), Cubozoa (box jellyfish), Hydrozoa (species, Hydra and Hydractinia), Anthozoa (sea anemone, corals, and sea pen), and Staurozoa (stalked jellyfish). Peptides play important roles as signaling molecules in development and differentiation in cnidaria. For example, cnidaria use peptides for cell-to cell communication. Recent discoveries show that Hydra neuropeptides control several biological processes including muscle contraction, neuron differentiation, and metamorphosis. Here, I describe the structure and functions of neuropeptides in Hydra and other cnidarian species. I also discuss that so-called primitive nervous system of Hydra is in more complex than generally believed. I also discuss how cnidaria use peptides for communication among cells rather than in higher animals.

Insight into the molecular and functional diversity of cnidarian neuropeptides

Cnidarians are the most primitive animals to possess a nervous system. This phylum is composed of the classes Scyphozoa (jellyfish), Cubozoa (box jellyfish), and Hydrozoa (e.g., Hydra, Hydractinia), which make up the subphylum Medusozoa, as well as the class Anthozoa (sea anemones and corals). Neuropeptides have an early evolutionary origin and are already abundant in cnidarians. For example, from the cnidarian Hydra, a key model system for studying the peptides involved in developmental and physiological processes, we identified a wide variety of novel neuropeptides from Hydra magnipapillata (the Hydra Peptide Project). Most of these peptides act directly on muscle cells and induce contraction and relaxation. Some peptides are involved in cell differentiation and morphogenesis. In this review, we describe FMRFamide-like peptides (FLPs), GLWamide-family peptides, and the neuropeptide Hym-355; FPQSFLPRGamide. Several hundred FLPs have been isolated from invertebrate animals such as cnidarians. GLWamide-family peptides function as signaling molecules in muscle contraction, metamorphosis, and settlement in cnidarians. Hym-355; FPQSFLPRGamide enhances neuronal differentiation in Hydra. Recently, GLWamide-family peptides and Hym-355; FPQSFLPRGamide were shown to trigger oocyte maturation and subsequent spawning in the hydrozoan jellyfish Cytaeis uchidae. These findings suggest the importance of these neuropeptides in both developmental and physiological processes.

Developing the anemone Aiptasia as a tractable model for cnidarian-dinoflagellate symbiosis: the transcriptome of aposymbiotic A. pallida

BACKGROUND

Coral reefs are hotspots of oceanic biodiversity, forming the foundation of ecosystems that are important both ecologically and for their direct practical impacts on humans. Corals are declining globally due to a number of stressors, including rising sea-surface temperatures and pollution; such stresses can lead to a breakdown of the essential symbiotic relationship between the coral host and its endosymbiotic dinoflagellates, a process known as coral bleaching. Although the environmental stresses causing this breakdown are largely known, the cellular mechanisms of symbiosis establishment, maintenance, and breakdown are still largely obscure. Investigating the symbiosis using an experimentally tractable model organism, such as the small sea anemone Aiptasia, should improve our understanding of exactly how the environmental stressors affect coral survival and growth.

RESULTS

We assembled the transcriptome of a clonal population of adult, aposymbiotic (dinoflagellate-free) Aiptasia pallida from ~208 million reads, yielding 58,018 contigs. We demonstrated that many of these contigs represent full-length or near-full-length transcripts that encode proteins similar to those from a diverse array of pathways in other organisms, including various metabolic enzymes, cytoskeletal proteins, and neuropeptide precursors. The contigs were annotated by sequence similarity, assigned GO terms, and scanned for conserved protein domains. We analyzed the frequency and types of single-nucleotide variants and estimated the size of the Aiptasia genome to be ~421 Mb. The contigs and annotations are available through NCBI (Transcription Shotgun Assembly database, accession numbers JV077153-JV134524) and at http://pringlelab.stanford.edu/projects.html.

CONCLUSIONS

The availability of an extensive transcriptome assembly for A. pallida will facilitate analyses of gene-expression changes, identification of proteins of interest, and other studies in this important emerging model system.

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.

Control of planula migration by LWamide and RFamide neuropeptides in Hydractinia echinata

Planula larvae of Hydractinia echinata (Cnidaria) settled on a substratum migrate toward light. We observed that planula migration is not a continuous process. Instead, it consists of repeating cycles of active migration (about 8 min on average) and inactive resting periods (about 26 min on average). This pattern of periodic migration is regulated by LWamide and RFamide neuropeptides. LWamide (10(-8) mol l(-1)) stimulates migration primarily by making the active periods longer, whereas RFamide (10(-7) mol l(-1)) inhibits migration by blocking the initiation and also shortening the length of the active periods. Since sensory neurons containing LWamides and RFamides are present in planula larvae, it appears likely that planula migration is regulated by the release of endogenous neuropeptides in response to environmental cues.

An endogenous peptide is involved in internal control of metamorphosis in the marine invertebrate Cassiopea xamachana (Cnidaria: Scyphozoa)

In the metagenetic life-cycle of the scyphozoan Cassiopea xamachana metamorphosis of planula-larvae or larva-like buds to polyps is triggered by specific external cues which are transmitted inside the larva or bud where internal signals finally coordinate the initiation of metamorphosis. This study deals with an endogenous metamorphosis inducer present in planulae and buds of Cassiopea. The inductive cue is localized in the basal part of the buds and can be characterized as a peptide with an apparent molecular weight of about 7,000 Da. Further purification was performed via reversed phase HPLC on a C18 column. Additional inhibitor assays revealed that protein kinase C and PI3 kinase, two known elements of the metamorphosis-inducing signal transduction cascade in Cassiopea, may act downstream of the endogenous inducing peptide.

Metamorphosis in the Cnidaria

The free-living stages of sedentary organisms are an adaptation that enables immobile species to exploit scattered or transient ecological niches. In the Cnidaria the task of prospecting for and identifying a congenial habitat is consigned to tiny planula larvae or larva-like buds, stages that actually transform into the sessile polyp. However, the sensory equipment of these larvae does not qualify them to locate an appropriate habitat from a distance. They therefore depend on a hierarchy of key stimuli indicative of an environment that is congenial to them; this is exemplified by genera of the Anthozoa (Nematostella, Acropora), Scyphozoa (Cassiopea), and Hydrozoa (Coryne, Proboscidactyla, Hydractinia). In many instances the final stimulus that triggers settlement and metamorphosis derives from substrate-borne bacteria or other biogenic cues which can be explored by mechanochemical sensory cells. Upon stimulation, the sensory cells release, or cause the release of, internal signals such as neuropeptides that can spread throughout the body, triggering decomposition of the larval tissue and acquisition of an adult cellular inventory. Progenitor cells may be preprogrammed to adopt their new tasks quickly. Gregarious settlement favours the exchange of alleles, but also can be a cause of civil war. A rare and spatially restricted substrate must be defended. Cnidarians are able to discriminate between isogeneic and allogeneic members of a community, and may use particular nematocysts to eliminate allogeneic competitors. Paradigms for most of the issues addressed are provided by the hydroid genus Hydractinia.

The hydroid Hydractinia: a versatile, informative cnidarian representative

The Cnidaria represent the most ancient eumetazoan phylum. Members of this group possess typical animal cells and tissues such as sensory cells, nerve cells, muscle cells and epithelia. Due to their unique phylogenetic position, cnidarians have traditionally been used as a reference group in various comparative studies. We propose the colonial marine hydroid, Hydractinia, as a convenient, versatile platform for basic and applied research in developmental biology, reproduction, immunology, environmental studies and more. In addition to being a typical cnidarian representative, Hydractinia offers many practical and theoretical advantages: studies that are feasible in Hydra like regeneration, pattern regulation, and cell renewal from stem cells, can be supplemented by genetic analyses and classical embryology in Hydractinia. Metamorphosis of the planula larva of Hydractinia can be used as a model for cell activation and communication and the presence of a genetically controlled allorecognition system makes it a suitable model for comparative immunology. Most importantly, Hydractinia may be manipulated at most aspects of its (short) life cycle. It has already been the subject of many studies in various disciplines, some of which are discussed in this essay.