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

Evolution of synapses and neurotransmitter systems: The divide-and-conquer model for early neural cell-type evolution

Nervous systems evolved around 560 million years ago to coordinate and empower animal bodies. Ctenophores - one of the earliest-branching lineages - are thought to share a few neuronal genes with bilaterians and may have evolved neurons convergently. Here we review our current understanding of the evolution of neuronal molecules in nonbilaterians. We also reanalyse single-cell sequencing data in light of new cell-cluster identities from a ctenophore and uncover evidence supporting the homology of one ctenophore neuron-type with neurons in Bilateria. The specific coexpression of the presynaptic proteins Unc13 and RIM with voltage-gated channels, neuropeptides and homeobox genes pinpoint a spiking sensory-peptidergic cell in the ctenophore mouth. Similar Unc13-RIM neurons may have been present in the first eumetazoans to rise to dominance only in stem Bilateria. We hypothesise that the Unc13-RIM lineage ancestrally innervated the mouth and conquered other parts of the body with the rise of macrophagy and predation during the Cambrian explosion.

Diel patterning in the bacterial community associated with the sea anemone Nematostella vectensis

Microbes can play an important role in the physiology of animals by providing essential nutrients, inducing immune pathways, and influencing the specific species that compose the microbiome through competitive or facilitatory interactions. The community of microbes associated with animals can be dynamic depending on the local environment, and factors that influence the composition of the microbiome are essential to our understanding of how microbes may influence the biology of their animal hosts. Regularly repeated changes in the environment, such as diel lighting, can result in two different organismal responses: a direct response to the presence and absence of exogenous light and endogenous rhythms resulting from a molecular circadian clock, both of which can influence the associated microbiota. Here, we report how diel lighting and a potential circadian clock impacts the diversity and relative abundance of bacteria in the model cnidarian Nematostella vectensis using an amplicon-based sequencing approach. Comparisons of bacterial communities associated with anemones cultured in constant darkness and in light:dark conditions revealed that individuals entrained in the dark had a more diverse microbiota. Overall community composition showed little variation over a 24-hr period in either treatment; however, abundances of individual bacterial OTUs showed significant cycling in each treatment. A comparative analysis of genes involved in the innate immune system of cnidarians showed differential expression between lighting conditions in N. vectensis, with significant up-regulation during long-term darkness for a subset of genes. Together, our studies support a hypothesis that the bacterial community associated with this species is relatively stable under diel light conditions when compared with static conditions and that particular bacterial members may have time-dependent abundance that coincides with the diel photoperiod in an otherwise stable community.

Transcriptome-wide analysis of differential gene expression in response to light:dark cycles in a model cnidarian

Animals respond to diurnal shifts in their environment with a combination of behavioral, physiological, and molecular changes to synchronize with regularly-timed external cues. Reproduction, movement, and metabolism in cnidarians have all been shown to be regulated by diurnal lighting, but the molecular mechanisms that may be responsible for these phenotypes remain largely unknown. The starlet sea anemone, Nematostella vectensis, has oscillating patterns of locomotion and respiration, as well as the molecular components of a putative circadian clock that may provide a mechanism for these light-induced responses. Here, we compare transcriptomic responses of N. vectensis when cultured under a diurnal lighting condition (12 h light: 12 h dark) with sea anemones cultured under constant darkness for 20 days. More than 3,000 genes (~13% of transcripts) had significant differences in expression between light and dark, with most genes having higher expression in the photoperiod. Following removal of the light cue 678 genes lost differential expression, suggesting that light-entrained gene expression by the circadian clock has temporal limits. Grouping of genes differentially expressed in light:dark conditions showed that cell cycle and transcription maintained diel expression in the absence of light, while many of the genes related to metabolism, antioxidants, immunity, and signal transduction lost differential expression without a light cue. Our data highlight the importance of diel light cycles on circadian mechanisms in this species, prompting new hypotheses for the role of photoreception in major biological processes, e.g., metabolism, immunity.

Canonical and cellular pathways timing gamete release in Acropora digitifera, Okinawa, Japan

Natural light cycles are important for synchronizing behavioural and physiological rhythms over varying time periods in both plants and animals. An endogenous clock, regulated by positive and negative elements, interacting in feedback loops controls these rhythms. Many corals exhibit diel cycles of polyp expansion and contraction entrained by solar light patterns and monthly cycles of spawning or planulation that correspond to nocturnal lunar light cycles. However, despite considerable interest in studies of coral reproduction, there is currently not enough molecular information about the cellular pathways involved with synchronizing spawning/planulation in broadcast spawners and brooders. To determine whether the endogenous clock is implicated in the regulation of reproductive behaviour in corals, we characterized the transcriptome of Acropora digitifera colonies at twelve time points over a 2-month period of full and new moons, starting with the day of spawning in June 2014. We identified 608 transcripts with differential expression only on the spawning night during the coral setting phase and gamete release. Our data revealed an upregulation of light-sensing molecules and rhodopsin-like receptors that initiate signalling cascades, including the glutamate, SMAD signalling and WNT signalling pathways, neuroactive ligand-receptor interactions and calcium signalling. These are all involved in cell cycling, cell movement, tissue polarity, focal adhesion and cytoskeleton reorganization and together lead to gamete release. These findings can improve the understanding of many time-based cycles and extend our knowledge of the interplay between exogenous signals and the endogenous clock in cnidarians.

Melatonin: exceeding expectations

Melatonin is a small, highly conserved indole with numerous receptor-mediated and receptor-independent actions. Receptor-dependent functions include circadian rhythm regulation, sleep, and cancer inhibition. The receptor-independent actions relate to melatonin's ability to function in the detoxification of free radicals, thereby protecting critical molecules from the destructive effects of oxidative stress under conditions of ischemia/reperfusion injury (stroke, heart attack), ionizing radiation, and drug toxicity, among others. Melatonin has numerous applications in physiology and medicine.

Developmental and light-entrained expression of melatonin and its relationship to the circadian clock in the sea anemone Nematostella vectensis

Background

The primary hormone of the vertebrate pineal gland, melatonin, has been identified broadly throughout the eukaryotes. While the role for melatonin in cyclic behavior via interactions with the circadian clock has only been reported in vertebrates, comparative research has shown that the transcription-translation loops of the animal circadian clock likely date to the cnidarian-bilaterian ancestor, leaving open significant questions about the evolutionary origin of melatonin signaling in circadian behavior by interacting with the molecular clock.

Results

Expression of melatonin in adult anemones showed peak expression at the end of light period (zeitgeber time (ZT) = 12) when cultured under diel conditions, coinciding with expression of genes and enzyme activity for members of the melatonin synthesis pathway (tryptophan hydroxylase and hydroxyindol-O-methyltransferase), which also showed rhythmic expression. During embryogenesis and juvenile stages, melatonin showed cyclic oscillations in concentration, peaking in midday. Spatial (in situ hybridization) and quantitative (real-time PCR) transcription of clock genes during development of N. vectensis showed these ‘clock’ genes are expressed early in the development, prior to rhythmic oscillations, suggesting functions independent of a function in the circadian clock. Finally, time-course studies revealed that animals transferred from diel conditions to constant darkness lose circadian expression for most of the clock genes within 4 days, which can be reset by melatonin supplementation.

Conclusions

Our results support an ancient role for melatonin in the circadian behavior of animals by showing cyclic expression of this hormone under diel conditions, light-dependent oscillations in genes in the melatonin synthesis pathway, and the function of melatonin in initiating expression of circadian clock genes in the cnidarian N. vectensis. The differences in expression melatonin and the circadian clock gene network in the adult stage when compared with developmental stages of N. vectensis suggests new research directions to characterize stage-specific mechanisms of circadian clock function in animals.

Melatonin, energy metabolism, and obesity: a review

Melatonin is an old and ubiquitous molecule in nature showing multiple mechanisms of action and functions in practically every living organism. In mammals, pineal melatonin functions as a hormone and a chronobiotic, playing a major role in the regulation of the circadian temporal internal order. The anti-obesogen and the weight-reducing effects of melatonin depend on several mechanisms and actions. Experimental evidence demonstrates that melatonin is necessary for the proper synthesis, secretion, and action of insulin. Melatonin acts by regulating GLUT4 expression and/or triggering, via its G-protein-coupled membrane receptors, the phosphorylation of the insulin receptor and its intracellular substrates mobilizing the insulin-signaling pathway. Melatonin is a powerful chronobiotic being responsible, in part, by the daily distribution of metabolic processes so that the activity/feeding phase of the day is associated with high insulin sensitivity, and the rest/fasting is synchronized to the insulin-resistant metabolic phase of the day. Furthermore, melatonin is responsible for the establishment of an adequate energy balance mainly by regulating energy flow to and from the stores and directly regulating the energy expenditure through the activation of brown adipose tissue and participating in the browning process of white adipose tissue. The reduction in melatonin production, as during aging, shift-work or illuminated environments during the night, induces insulin resistance, glucose intolerance, sleep disturbance, and metabolic circadian disorganization characterizing a state of chronodisruption leading to obesity. The available evidence supports the suggestion that melatonin replacement therapy might contribute to restore a more healthy state of the organism.

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.

Occurrence, diel patterns, and the influence of melatonin on the photosynthetic performance of cultured Symbiodinium

Dinoflagellata is the earliest phylum in which true circadian regulation of melatonin rhythms has been convincingly demonstrated. Here, diel profiling of melatonin in a cultured member of this phylum belonging to the genus Symbiodinium indicated that melatonin levels oscillate with significant nocturnal peaks. However, unlike in other previously studied dinoflagellate species, the diel rhythmicity of melatonin in Symbiodinium did not persist under constant dark conditions. Thus, the oscillating pattern of melatonin in Symbiodinium is presumed not to be driven by endogenous circadian control of melatonin production, but rather by changes in the daily photocycle, most likely through a mechanism involving the enhanced photo-consumption of melatonin by free radicals. Although direct interactions of melatonin with detrimental radicals have been previously studied in several basal species, including dinoflagellates, none of these investigations addressed the effects that this molecule may have on photosynthesis, a major source of radical species in unicellular algae. In the present work, real-time monitoring of oxygen evolution in Symbiodinium cultures indicated a significant decrease in photosynthesis rates upon treatment with various doses of melatonin. Analyses of chlorophyll a fluorescence and xanthophyll cycle activity confirmed this effect and further revealed that this slowdown may occur through an enhanced engagement of photoprotective mechanisms in melatonin-treated cells. These findings are of great importance as they demonstrate that in certain photoautotroph species, the interactions of elevated melatonin levels with photosynthesis may extend beyond the general purpose of antioxidant protection.

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.