GABAergic synaptic transmission modulates swimming in the ascidian larva

Development and circuitry of the tunicate larval Motor Ganglion, a putative hindbrain/spinal cord homolog

The Motor Ganglion (MG) is a small collection of neurons that control the swimming movements of the tunicate tadpole larva. Situated at the base of the tail, molecular and functional comparisons suggest that may be a homolog of the spinal cord and/or hindbrain ("rhombospinal" region) of vertebrates. Here we review the most current knowledge of the development, connectivity, functions, and unique identities of the neurons that comprise the MG, drawn mostly from studies in Ciona spp. The simple cell lineages, minimal cellular composition, and comprehensively mapped "connectome" of the Ciona MG all make this an excellent model for studying the development and physiology of motor control in aquatic larvae.

Polymodal sensory perception drives settlement and metamorphosis of Ciona larvae

The Earth's oceans brim with an incredible diversity of microscopic lifeforms, including motile planktonic larvae, whose survival critically depends on effective dispersal in the water column and subsequent exploration of the seafloor to identify a suitable settlement site. How their nervous systems mediate sensing of diverse multimodal cues remains enigmatic. Here, we uncover that the tunicate Ciona intestinalis larvae employ ectodermal sensory cells to sense various mechanical and chemical cues. Combining whole-brain imaging and chemogenetics, we demonstrate that stimuli encoded at the periphery are sufficient to drive global brain-state changes to promote or impede both larval attachment and metamorphosis behaviors. The ability of C. intestinalis larvae to leverage polymodal sensory perception to support information coding and chemotactile behaviors may explain how marine larvae make complex decisions despite streamlined nervous systems.

The neuroendocrine system of Ciona intestinalis Type A, a deuterostome invertebrate and the closest relative of vertebrates

Deuterostome invertebrates, including echinoderms, hemichordates, cephalochordates, and urochordates, exhibit common and species-specific morphological, developmental, physiological, and behavioral characteristics that are regulated by neuroendocrine and nervous systems. Over the past 15 years, omics, genetic, and/or physiological studies on deuterostome invertebrates have identified low-molecular-weight transmitters, neuropeptides and their cognate receptors, and have clarified their various biological functions. In particular, there has been increasing interest on the neuroendocrine and nervous systems of Ciona intestinalis Type A, which belongs to the subphylum Urochordata and occupies the critical phylogenetic position as the closest relative of vertebrates. During the developmental stage, gamma-aminobutylic acid, D-serine, and gonadotropin-releasing hormones regulate metamorphosis of Ciona. In adults, the neuropeptidergic mechanisms underlying ovarian follicle growth, oocyte maturation, and ovulation have been elucidated. This review article provides the most recent and fundamental knowledge of the neuroendocrine and nervous systems of Ciona, and their evolutionary aspects.

Comprehensive analysis of locomotion dynamics in the protochordate Ciona intestinalis reveals how neuromodulators flexibly shape its behavioral repertoire

Vertebrate nervous systems can generate a remarkable diversity of behaviors. However, our understanding of how behaviors may have evolved in the chordate lineage is limited by the lack of neuroethological studies leveraging our closest invertebrate relatives. Here, we combine high-throughput video acquisition with pharmacological perturbations of bioamine signaling to systematically reveal the global structure of the motor behavioral repertoire in the Ciona intestinalis larvae. Most of Ciona’s postural variance can be captured by 6 basic shapes, which we term “eigencionas.” Motif analysis of postural time series revealed numerous stereotyped behavioral maneuvers including “startle-like” and “beat-and-glide.” Employing computational modeling of swimming dynamics and spatiotemporal embedding of postural features revealed that behavioral differences are generated at the levels of motor modules and the transitions between, which may in part be modulated by bioamines. Finally, we show that flexible motor module usage gives rise to diverse behaviors in response to different light stimuli.

Vertebrate nervous systems can generate a remarkable diversity of behaviors, but how did these evolve in the chordate lineage? A study of the protochordate Ciona intestinalis reveals novel insights into how a simple chordate brain uses neuromodulators to control its behavioral repertoire.

The trunk-tail junctional region in Ciona larvae autonomously expresses tail-beating bursts at ∼20 second intervals

Swimming locomotion in aquatic vertebrates, such as fish and tadpoles, is expressed through neuron networks in the spinal cord. These networks are arranged in parallel, ubiquitously distributed and mutually coupled along the spinal cord to express undulation patterns accommodated to various inputs into the networks. While these systems have been widely studied in vertebrate swimmers, their evolutionary origin along the chordate phylogeny remains unclear. Ascidians, representing a sister group of vertebrates, give rise to tadpole larvae that swim freely in seawater. In the present study, we examined the locomotor ability of the anterior and posterior body fragments of larvae of the ascidian Ciona that had been cut at an arbitrary position. Examination of more than 200 fragments revealed a necessary and sufficient body region that spanned only ∼10% of the body length and included the trunk-tail junction. 'Mid-piece' body fragments, which included the trunk-tail junctional region, but excluded most of the anterior trunk and posterior tail, autonomously expressed periodic tail-beating bursts at ∼20 s intervals. We compared the durations and intervals of tail-beating bursts expressed by mid-piece fragments, and also by whole larvae under different sensory conditions. The results suggest that body parts outside the mid-piece effect shortening of swimming intervals, particularly in the dark, and vary the burst duration. We propose that Ciona larvae express swimming behaviors by modifying autonomous and periodic locomotor drives that operate locally in the trunk-tail junctional region.

Molecular Characterization and Expression Analysis of Putative Class C (Glutamate Family) G Protein-Coupled Receptors in Ascidian Styela clava

In this study, we performed the genome-wide domain analysis and sequence alignment on the genome of Styela clava, and obtained a repertoire of 204 putative GPCRs, which exhibited a highly reduced gene number compared to vertebrates and cephalochordates. In this repertoire, six Class C GPCRs, including four metabotropic glutamate receptors (Sc-GRMs), one calcium-sensing receptor (Sc-CaSR), and one gamma-aminobutyric acid (GABA) type B receptor 2-like (Sc-GABABR2-like) were identified, with the absence of type 1 taste and vomeronasal receptors. All the Sc-GRMs and Sc-CaSR contained the typical "Venus flytrap" and cysteine-rich domains required for ligand binding and subsequent propagation of conformational changes. In swimming larvae, Sc-grm3 and Sc-casr were mainly expressed at the junction of the sensory vesicle and tail nerve cord while the transcripts of Sc-grm4, Sc-grm7a, and Sc-grm7b appeared at the anterior trunk, which suggested their important functions in neurotransmission. The high expression of these Class C receptors at tail-regression and metamorphic juvenile stages hinted at their potential involvement in regulating metamorphosis. In adults, the transcripts were highly expressed in several peripheral tissues, raising the possibility that S. clava Class C GPCRs might function as neurotransmission modulators peripherally after metamorphosis. Our study systematically characterized the ancestral chordate Class C GPCRs to provide insights into the origin and evolution of these receptors in chordates and their roles in regulating physiological and morphogenetic changes relevant to the development and environmental adaption.

Novel Insights on Nitric Oxide Synthase and NO Signaling in Ascidian Metamorphosis

Nitric oxide (NO) is a pivotal signaling molecule involved in a wide range of physiological and pathological processes. We investigated NOS/NO localization patterns during the different stages of larval development in the ascidia Ciona robusta and evidenced a specific and temporally controlled pattern. NOS/NO expression starts in the most anterior sensory structures of the early larva and progressively moves towards the caudal portion as larval development and metamorphosis proceeds. We here highlight the pattern of NOS/NO expression in the central and peripheral nervous system of Ciona larvae which precisely follows the progression of neural signals of the central pattern generator necessary for the control of the movements of the larva towards the substrate. This highly dynamic localization profile perfectly matches with the central role played by NO from the first phase of settlement induction to the next control of swimming behavior, adhesion to substrate and progressive tissue resorption and reorganization of metamorphosis itself.

Fluorescence Properties of a Novel Isoquinoline Derivative Tested in an Invertebrate Chordate, Ciona intestinalis

3‐Hydroxyisoquinolines (ISOs) and their tautomeric isoquinolin‐3‐ones are heterocycles with attractive biological properties. Here we reported the revisited synthesis of a highly functionalized ISO that showed blue fluorescence and the characterization of its biological properties in an invertebrate animal model, the ascidian Ciona intestinalis. Larvae exposed to ISO at concentrations higher than 1 μM showed an intense fluorescence localized in the cell nuclei of all tissues. Moreover, exposure to ISO interfered with larval ability to swim; this neuromuscular effect was reversible. Overall, these results suggested that ISOs can have promising applications as novel fluorescent dyes of the cell nuclei.

Toxicity of nanoplastics during the embryogenesis of the ascidian Ciona robusta (Phylum Chordata)

Nanoplastics are considered contaminants of emerging concern at the global scale. The recent evidence of their occurrence in seawater from the Mediterranean Sea calls for a thorough evaluation of their impact on marine life and in particular on vulnerable life stages such as planktonic embryos. Here, we investigated the impact of increasing nominal concentrations of 50 nm amino-modified (PS-NH₂) and 60 nm carboxy-modified (PS-COOH) polystyrene nanoparticles (PS NPs) on the embryonic development of the ascidian Ciona robusta (phylum Chordata), a common benthic invertebrate living in Mediterranean coastal areas with the peculiarity of being an early chordate developmental model. A strong agglomeration of PS-COOH (approx. 1 µm) was observed in natural sea water (NSW) already at time 0, while PS-NH₂ resulted still monodispersed (approx. 130 nm) but largely aggregated after 22 h with a microscale dimension similar to those negatively charged. However, their effect on C. robusta embryos development largely differed at 22 h: PS-COOH did not affect larvae phenotypes nor their development, while PS-NH₂ caused a dose-dependent effect (EC₅₀ (22 h) of 7.52 μg mL^-1) with various degrees of phenotype malformations (from mild to severe) and impairment of larval swimming. Embryos (up to 30%) exposed to 15 µg mL^-1PS-NH₂ resulted not developed and the majority was unable to hatch. Calculated PS-NH₂ EC₅₀ resulted higher than those available for other marine invertebrate species, suggesting a protective role of the egg envelopes surrounding C. robusta embryos toward nanoplastics exposure.

Effector gene expression underlying neuron subtype-specific traits in the Motor Ganglion of Ciona

The central nervous system of the Ciona larva contains only 177 neurons. The precise regulation of neuron subtype-specific morphogenesis and differentiation observed during the formation of this minimal connectome offers a unique opportunity to dissect gene regulatory networks underlying chordate neurodevelopment. Here we compare the transcriptomes of two very distinct neuron types in the hindbrain/spinal cord homolog of Ciona, the Motor Ganglion (MG): the Descending decussating neuron (ddN, proposed homolog of Mauthner Cells in vertebrates) and the MG Interneuron 2 (MGIN2). Both types are invariantly represented by a single bilaterally symmetric left/right pair of cells in every larva. Supernumerary ddNs and MGIN2s were generated in synchronized embryos and isolated by fluorescence-activated cell sorting for transcriptome profiling. Differential gene expression analysis revealed ddN- and MGIN2-specific enrichment of a wide range of genes, including many encoding potential "effectors" of subtype-specific morphological and functional traits. More specifically, we identified the upregulation of centrosome-associated, microtubule-stabilizing/bundling proteins and extracellular guidance cues part of a single intrinsic regulatory program that might underlie the unique polarization of the ddNs, the only descending MG neurons that cross the midline. Consistent with our predictions, CRISPR/Cas9-mediated, tissue-specific elimination of two such candidate effectors, Efcab6-related and Netrin1, impaired ddN polarized axon outgrowth across the midline.

Regulation and evolution of muscle development in tunicates

For more than a century, studies on tunicate muscle formation have revealed many principles of cell fate specification, gene regulation, morphogenesis, and evolution. Here, we review the key studies that have probed the development of all the various muscle cell types in a wide variety of tunicate species. We seize this occasion to explore the implications and questions raised by these findings in the broader context of muscle evolution in chordates.

Parallel visual circuitry in a basal chordate

A common CNS architecture is observed in all chordates, from vertebrates to basal chordates like the ascidian Ciona. Ciona stands apart among chordates in having a complete larval connectome. Starting with visuomotor circuits predicted by the Ciona connectome, we used expression maps of neurotransmitter use with behavioral assays to identify two parallel visuomotor circuits that are responsive to different components of visual stimuli. The first circuit is characterized by glutamatergic photoreceptors and responds to the direction of light. These photoreceptors project to cholinergic motor neurons, via two tiers of cholinergic interneurons. The second circuit responds to changes in ambient light and mediates an escape response. This circuit uses GABAergic photoreceptors which project to GABAergic interneurons, and then to cholinergic interneurons. Our observations on the behavior of larvae either treated with a GABA receptor antagonist or carrying a mutation that eliminates photoreceptors indicate the second circuit is disinhibitory.

Developmental signature, synaptic connectivity and neurotransmission are conserved between vertebrate hair cells and tunicate coronal cells

In tunicates, the coronal organ represents a sentinel checking particle entrance into the pharynx. The organ differentiates from an anterior embryonic area considered a proto-placode. For their embryonic origin, morphological features and function, coronal sensory cells have been hypothesized to be homologues to vertebrate hair cells. However, vertebrate hair cells derive from a posterior placode. This contradicts one of the principle historical criteria for homology, similarity of position, which could be taken as evidence against coronal cells/hair cells homology. In the tunicates Ciona intestinalis and C. robusta, we found that the coronal organ expresses genes (Atoh, Notch, Delta-like, Hairy-b, and Musashi) characterizing vertebrate neural and hair cell development. Moreover, coronal cells exhibit a complex synaptic connectivity pattern, and express neurotransmitters (Glu, ACh, GABA, 5-HT, and catecholamines), or enzymes for their synthetic machinery, involved in hair cell activity. Lastly, coronal cells express the Trpa gene, which encodes an ion channel expressed in hair cells. These data lead us to hypothesize a model in which competence to make secondary mechanoreceptors was initially broadly distributed through placode territories, but has become confined to different placodes during the evolution of the vertebrate and tunicate lineages.

Morphology and Physiology of the Ascidian Nervous Systems and the Effectors

Neurobiology in ascidians has made many advances. Ascidians have offered natural advantages to researchers, including fecundity, structural simplicity, invariant morphology, and fast and stereotyped developmental processes. The researchers have also accumulated on this animal a great deal of knowledge, genomic resources, and modern genetic techniques. A recent connectomic analysis has shown an ultimately resolved image of the larval nervous system, whereas recent applications of live imaging and optogenetics have clarified the functional organization of the juvenile nervous system. Progress in resources and techniques have provided convincing ways to deepen what we have wanted to know about the nervous systems of ascidians. Here, the research history and the current views regarding ascidian nervous systems are summarized.

The peripheral nervous system of the ascidian tadpole larva: Types of neurons and their synaptic networks

Physical and chemical cues from the environment are used to direct animal behavior through a complex network of connections originating in exteroceptors. In chordates, mechanosensory and chemosensory neurons of the peripheral nervous system (PNS) must signal to the motor circuits of the central nervous system (CNS) through a series of pathways that integrate and regulate the output to motor neurons (MN); ultimately these drive contraction of the tail and limb muscles. We used serial-section electron microscopy to reconstruct PNS neurons and their hitherto unknown synaptic networks in the tadpole larva of a sibling chordate, the ascidian, Ciona intestinalis. The larva has groups of neurons in its apical papillae, epidermal neurons in the rostral and apical trunk, caudal neurons in the dorsal and ventral epidermis, and a single tail tip neuron. The connectome reveals that the PNS input arises from scattered groups of these epidermal neurons, 54 in total, and has three main centers of integration in the CNS: in the anterior brain vesicle (which additionally receives input from photoreceptors of the ocellus), the motor ganglion (which contains five pairs of MN), and the tail, all of which in turn are themselves interconnected through important functional relay neurons. Some neurons have long collaterals that form autapses. Our study reveals interconnections with other sensory systems, and the exact inputs to the motor system required to regulate contractions in the tail that underlie larval swimming, or to the CNS to regulate substrate preference prior to the induction of larval settlement and metamorphosis.

Roles of Retinoic Acid Signaling in Shaping the Neuronal Architecture of the Developing Amphioxus Nervous System

The morphogen retinoic acid (RA) patterns vertebrate nervous systems and drives neurogenesis, but how these functions evolved remains elusive. Here, we show that RA signaling plays stage- and tissue-specific roles during the formation of neural cell populations with serotonin, dopamine, and GABA neurotransmitter phenotypes in amphioxus, a proxy for the ancestral chordate. Our data suggest that RA signaling restricts the specification of dopamine-containing cells in the ectoderm and of GABA neurons in the neural tube, probably by regulating Hox1 and Hox3 gene expression, respectively. The two Hox genes thus appear to serve distinct functions rather than to participate in a combinatorial Hox code. We were further able to correlate the RA signaling-dependent mispatterning of hindbrain GABA neurons with concomitant motor impairments. Taken together, these data provide new insights into how RA signaling and Hox genes contribute to nervous system as well as to motor control development in amphioxus and hence shed light on the evolution of these functions within vertebrates.

Perchance to dream? Primordial motor activity patterns in vertebrates from fish to mammals: their prenatal origin, postnatal persistence during sleep, and pathological reemergence during REM sleep behavior disorder

An overview is presented of the literature dealing with sleep-like motility and concomitant neuronal activity patterns throughout the life cycle in vertebrates, ectothermic as well as endothermic. Spontaneous, periodically modulated, neurogenic bursts of non-purposive movements are a universal feature of larval and prenatal behavior, which in endothermic animals (i.e. birds and mammals) continue to occur periodically throughout life. Since the entire body musculature is involved in ever-shifting combinations, it is proposed that these spontaneously active periods be designated as 'rapid-BODY-movement' (RBM) sleep. The term 'rapid-EYE-movement (REM) sleep', characterized by attenuated muscle contractions and reduced tonus, can then be reserved for sleep at later stages of development. Mature stages of development in which sustained muscle atonia is combined with 'paradoxical arousal' of cortical neuronal firing patterns indisputably represent the evolutionarily most recent aspect of REM sleep, but more research with ectothermic vertebrates, such as fish, amphibians and reptiles, is needed before it can be concluded (as many prematurely have) that RBM is absent in these species. Evidence suggests a link between RBM sleep in early development and the clinical condition known as 'REM sleep behavior disorder (RBD)', which is characterized by the resurgence of periodic bouts of quasi-fetal motility that closely resemble RBM sleep. Early developmental neuromotor risk factors for RBD in humans also point to a relationship between RBM sleep and RBD.

An endocrine disruptor, bisphenol A, affects development in the protochordate Ciona intestinalis: hatching rates and swimming behavior alter in a dose-dependent manner

Bisphenol A (BPA) is widely used industrially to produce polycarbonate plastics and epoxy resins. Numerous studies document the harmful effects caused by low-dose BPA exposure especially on nervous systems and behavior in experimental animals such as mice and rats. Here, we exposed embryos of a model chordate, Ciona intestinalis, to seawater containing BPA to evaluate adverse effects on embryonic development and on the swimming behavior of subsequent larvae. Ciona is ideal because its larva develops rapidly and has few cells. The rate of larval hatching decreased in a dose-dependent manner with exposures to BPA above 3 μM; swimming behavior was also affected in larvae emerging from embryos exposed to 1 μM BPA. Adverse effects were most severe on fertilized eggs exposed to BPA within 7 h post-fertilization. Ciona shares twelve nuclear receptors with mammals, and BPA is proposed to disturb the physiological functions of one or more of these.

Monoaminergic modulation of photoreception in ascidian: evidence for a proto-hypothalamo-retinal territory

Background

The retina of craniates/vertebrates has been proposed to derive from a photoreceptor prosencephalic territory in ancestral chordates, but the evolutionary origin of the different cell types making the retina is disputed. Except for photoreceptors, the existence of homologs of retinal cells remains uncertain outside vertebrates.

Methods

The expression of genes expressed in the sensory vesicle of the ascidian Ciona intestinalis including those encoding components of the monoaminergic neurotransmission systems, was analyzed by in situ hybridization or in vivo transfection of the corresponding regulatory elements driving fluorescent reporters. Modulation of photic responses by monoamines was studied by electrophysiology combined with pharmacological treatments.

Results

We show that many molecular characteristics of dopamine-synthesizing cells located in the vicinity of photoreceptors in the sensory vesicle of the ascidian Ciona intestinalis are similar to those of amacrine dopamine cells of the vertebrate retina. The ascidian dopamine cells share with vertebrate amacrine cells the expression of the key-transcription factor Ptf1a, as well as that of dopamine-synthesizing enzymes. Surprisingly, the ascidian dopamine cells accumulate serotonin via a functional serotonin transporter, as some amacrine cells also do. Moreover, dopamine cells located in the vicinity of the photoreceptors modulate the light-off induced swimming behavior of ascidian larvae by acting on alpha2-like receptors, instead of dopamine receptors, supporting a role in the modulation of the photic response. These cells are located in a territory of the ascidian sensory vesicle expressing genes found both in the retina and the hypothalamus of vertebrates (six3/6, Rx, meis, pax6, visual cycle proteins).

Conclusion

We propose that the dopamine cells of the ascidian larva derive from an ancestral multifunctional cell population located in the periventricular, photoreceptive field of the anterior neural tube of chordates, which also gives rise to both anterior hypothalamus and the retina in craniates/vertebrates. It also shows that the existence of multiple cell types associated with photic responses predates the formation of the vertebrate retina.

Neural tube patterning by Ephrin, FGF and Notch signaling relays

The motor ganglion (MG) controls the rhythmic swimming behavior of the Ciona intestinalis tadpole. Despite its cellular simplicity (five pairs of neurons), the MG exhibits conservation of transcription factor expression with the spinal cord of vertebrates. Evidence is presented that the developing MG is patterned by sequential Ephrin/FGF/MAPK and Delta/Notch signaling events. FGF/MAPK attenuation by a localized EphrinAb signal specifies posterior neuronal subtypes, which in turn relay a Delta2/Notch signal that specifies anterior fates. This short-range relay is distinct from the patterning of the vertebrate spinal cord, which is a result of opposing BMP and Shh morphogen gradients. Nonetheless, both mechanisms lead to localized expression of related homeodomain codes for the specification of distinct neuronal subtypes. This MG regulatory network provides a foundation for elucidating the genetic and cellular basis of a model chordate central pattern generator.

Ion channels in key marine invertebrates; their diversity and potential for applications in biotechnology

Of the intra-membrane proteins, the class that comprises voltage and ligand-gated ion channels represents the major substrate whereby signals pass between and within cells in all organisms. It has been presumed that vertebrate and particularly mammalian ion channels represent the apex of evolutionary complexity and diversity and much effort has been focused on understanding their function. However, the recent availability of cheap high throughput genome sequencing has massively broadened and deepened the quality of information across phylogeny and is radically changing this view. Here we review current knowledge on such channels in key marine invertebrates where physiological evidence is backed up by molecular sequences and expression/functional studies. As marine invertebrates represent a much greater range of phyla than terrestrial vertebrates and invertebrates together, we argue that these animals represent a highly divergent, though relatively underused source of channel novelty. As ion channels are exquisitely selective sensors for voltage and ligands, their potential and actual applications in biotechnology are manifold.

Neuronal subtype specification in the spinal cord of a protovertebrate

The visceral ganglion (VG) comprises the basic motor pool of the swimming ascidian tadpole and has been proposed to be homologous to the spinal cord of vertebrates. Here, we use cis-regulatory modules, or enhancers, from transcription factor genes expressed in single VG neuronal precursors to label and identify morphologically distinct moto- and interneuron subtypes in the Ciona intestinalis tadpole larva. We also show that the transcription factor complement present in each differentiating neuron correlates with its unique morphology. Forced expression of putative interneuron markers Dmbx and Vsx results in ectopic interneuron-like cells at the expense of motoneurons. Furthermore, by perturbing upstream signaling events, we can change the transcription factor expression profile and subsequent identity of the different precursors. Perturbation of FGF signaling transforms the entire VG into Vsx+/Pitx+ putative cholinergic interneurons, while perturbation of Notch signaling results in duplication of Dmbx+ decussating interneurons. These experiments demonstrate the connection between transcriptional regulation and the neuronal subtype diversity underlying swimming behavior in a simple chordate.

Neuropeptides, hormone peptides, and their receptors in Ciona intestinalis: an update

The critical phylogenetic position of ascidians leads to the presumption that neuropeptides and hormones in vertebrates are highly likely to be evolutionarily conserved in ascidians, and the cosmopolitan species Ciona intestinalis is expected to be an excellent deuterostome Invertebrate model for studies on neuropeptides and hormones. Nevertheless, molecular and functional characterization of Ciona neuropeptides and hormone peptides was restricted to a few peptides such as a cholecystokinin (CCK)/gastrin peptide, cionin, and gonadotropin-releasing hormones (GnRHs). In the past few years, mass spectrometric analyses and database searches have detected Ciona orthologs or prototypes of vertebrate peptides and their receptors, including tachykinin, insulin/relaxin, calcitonin, and vasopressin. Furthermore, studies have shown that several Ciona peptides, including vasopressin and a novel GnRH-related peptide, have acquired ascidian-specific molecular forms and/or biological functions. These findings provided indisputable evidence that ascidians, unlike other invertebrates (including the traditional protostome model animals), possess neuropeptides and hormone peptides structurally and functionally related to vertebrate counterparts, and that several peptides have uniquely diverged in ascidian evolutionary lineages. Moreover, recent functional analyses of Ciona tachykinin in the ovary substantiated the novel tachykininergic protease-assoclated oocyte growth pathway, which could not have been revealed in studies on vertebrates. These findings confirm the outstanding advantages of ascidians in understanding the neuroscience, endocrinology, and evolution of vertebrate neuropeptides and hormone peptides. This article provides an overview of basic findings and reviews new knowledge on ascidian neuropeptides and hormone peptides.

Simple motor system of the ascidian larva: neuronal complex comprising putative cholinergic and GABAergic/glycinergic neurons

The ascidian larva is an excellent model for studies of the functional organization and neuronal circuits of chordates due to its remarkably simple central nervous system (CNS), comprised of about 100 neurons. To date, however, the identities of the various neurons in the ascidian larva, particularly their neurotransmitter phenotypes, are not well established. Acetylcholine, GABA, and glycine are critical neurotransmitters for locomotion in many animals. We visualized putative cholinergic neurons and GABAergic/glycinergic neurons in the ascidian larva by immunofluorescent staining using antibodies against vesicular acetylcholine transporter (VACHT) and vesicular GABA/glycine transporter (VGAT), respectively. Neurons expressing a cholinergic phenotype were found in the brain vesicle and the visceral ganglion. Five pairs of VACHT-positive neurons were located in the visceral ganglion. These putative cholinergic neurons extended their axons posteriorly and formed nerve terminals proximal to the most anterior muscle cells in the tail. VGAT-positive neurons were located in the brain vesicle, the visceral ganglion, and the anterior nerve cord. Two distinct pairs of VGAT-positive neurons, bilaterally aligned along the anterior nerve cord, extended axons anteriorly, near to the axons of the contralateral VACHT-positive neurons. Cell bodies of the VGAT-positive neurons lay on these nerve tracts. The neuronal complex, comprising motor neurons with a cholinergic phenotype and some of the GABA/glycinergic interneurons, has structural features that are compatible with a central pattern generator (CPG) producing a rhythmic movement of the tail. The simple CPG of the ascidian larva may represent the ancestral state of the vertebrate motor system.

Limited functions of Hox genes in the larval development of the ascidian Ciona intestinalis

In animals, region specific morphological characters along the anteroposterior axis are controlled by a number of developmental genes, including Hox genes encoding homeodomain transcription factors. Although Hox genes have been regarded to play a key role in the evolution of morphological diversity, as well as in the establishment of the body plan, little is known about the function of Hox genes in invertebrates, except for in insects and nematodes. The present study addresses the role of Hox genes in body patterning during the larval development of the ascidian Ciona intestinalis conducting knockdown experiments of the seven Hox genes expressed during embryogenesis. Experimental results have demonstrated that Ci-Hox12 plays an important role in tail development through the maintenance of expression of Ci-Fgf8/17/18 and Ci-Wnt5 in the tail tip epidermis. Additionally, it has been shown that Ci-Hox10 is involved in the development of GABAergic neurons in the dorsal visceral ganglion. Surprisingly, knockdown of Ci-Hox1, Ci-Hox2, Ci-Hox3, Ci-Hox4 and Ci-Hox5 did not give rise to any consistent morphological defects in the larvae. Furthermore, expression of neuronal marker genes was not affected in larvae injected with MOs against Ci-Hox1, Ci-Hox3 or Ci-Hox5. In conclusion, we suggest that the contribution of Hox genes to the larval development of the ascidian C. intestinalis might be limited, despite the fact that Ci-Hox10 and Ci-Hox12 play important roles in neuronal and tail development.

A glycine receptor is involved in the organization of swimming movements in an invertebrate chordate

BACKGROUND

Rhythmic motor patterns for locomotion in vertebrates are generated in spinal cord neural networks known as spinal Central Pattern Generators (CPGs). A key element in pattern generation is the role of glycinergic synaptic transmission by interneurons that cross the cord midline and inhibit contralaterally-located excitatory neurons. The glycinergic inhibitory drive permits alternating and precisely timed motor output during locomotion such as walking or swimming. To understand better the evolution of this system we examined the physiology of the neural network controlling swimming in an invertebrate chordate relative of vertebrates, the ascidian larva Ciona intestinalis.

RESULTS

A reduced preparation of the larva consisting of nerve cord and motor ganglion generates alternating swimming movements. Pharmacological and genetic manipulation of glycine receptors shows that they are implicated in the control of these locomotory movements. Morphological molecular techniques and heterologous expression experiments revealed that glycine receptors are inhibitory and are present on both motoneurones and locomotory muscle while putative glycinergic interneurons were identified in the nerve cord by labeling with an anti-glycine antibody.

CONCLUSIONS

In Ciona intestinalis, glycine receptors, glycinergic transmission and putative glycinergic interneurons, have a key role in coordinating swimming movements through a simple CPG that is present in the motor ganglion and nerve cord. Thus, the strong association between glycine receptors and vertebrate locomotory networks may now be extended to include the phylum chordata. The results suggest that the basic network for 'spinal-like' locomotion is likely to have existed in the common ancestor of extant chordates some 650 M years ago.

Cell type and function of neurons in the ascidian nervous system

Ascidians, or sea squirts, are primitive chordates, and their tadpole larvae share a basic body plan with vertebrates, including a notochord and a dorsal tubular central nervous system (CNS). The CNS of the ascidian larva is formed through a process similar to vertebrate neurulation, while the ascidian CNS is remarkably simple, consisting of about 100 neurons. Recent identification of genes that are specifically expressed in a particular subtype of neurons has enabled us to reveal neuronal networks at single-cell resolution. Based on the information on neuron subtype-specific genes, different populations of neurons have been visualized by whole-mount in situ hybridization, immunohistochemical staining using specific antibodies, and fluorescence labeling of cell bodies and neurites by a fluorescence protein reporter driven by neuron-specific promoters. Neuronal populations that have been successfully visualized include glutamatergic, cholinergic, gamma-aminobutyric acid/glycinergic, and dopaminergic neurons, which have allowed us to propose functional regionalization of the CNS and a neural circuit for locomotion. Thus, the simple nervous system of the ascidian larva can serve as an attractive model system for studying the development, function, and evolution of the chordate nervous system.

Immunohistochemical analysis of adhesive papillae of Clavelina lepadiformis (Müller, 1776) and Clavelina phlegraea (Salfi, 1929) (Tunicata, Ascidiacea)

Almost all ascidian larvae bear three mucus secreting and sensory organs, the adhesive papillae, at the anterior end of the trunk, which play an important role during the settlement phase. The morphology and the cellular composition of these organs varies greatly in the different species. The larvae of the Clavelina genus bear simple bulbous papillae, which are considered to have only a secretory function. We analysed the adhesive papillae of two species belonging to this genus, C. lepadiformis and C. phlegraea, by histological sections and by immunolocalisation of β-tubulin and serotonin, in order to better clarify the cellular composition of these organs. We demonstrated that they contain at least two types of neurons: central neurons, bearing microvilli, and peripheral ciliated neurons. Peripheral neurons of C. lepadiformis contain serotonin. We suggest that these two neurons play different roles during settlement: the central ones may be chemo- or mechanoreceptors that sense the substratum, and the peripheral ones may be involved in the mechanism that triggers metamorphosis.

Natural variation of model mutant phenotypes in Ciona intestinalis

BACKGROUND

The study of ascidians (Chordata, Tunicata) has made a considerable contribution to our understanding of the origin and evolution of basal chordates. To provide further information to support forward genetics in Ciona intestinalis, we used a combination of natural variation and neutral population genetics as an approach for the systematic identification of new mutations. In addition to the significance of developmental variation for phenotype-driven studies, this approach can encompass important implications in evolutionary and population biology.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we report a preliminary survey for naturally occurring mutations in three geographically interconnected populations of C. intestinalis. The influence of historical, geographical and environmental factors on the distribution of abnormal phenotypes was assessed by means of 12 microsatellites. We identified 37 possible mutant loci with stereotyped defects in embryonic development that segregate in a way typical of recessive alleles. Local populations were found to differ in genetic organization and frequency distribution of phenotypic classes.

CONCLUSIONS/SIGNIFICANCE

Natural genetic polymorphism of C. intestinalis constitutes a valuable source of phenotypes for studying embryonic development in ascidians. Correlating genetic structure and the occurrence of abnormal phenotypes is a crucial focus for understanding the selective forces that shape natural finite populations, and may provide insights of great importance into the evolutionary mechanisms that generate animal diversity.

Developmental expression of glutamic acid decarboxylase and of gamma-aminobutyric acid type B receptors in the ascidian Ciona intestinalis

We describe Ciona intestinalis gamma-aminobutyric acid (GABA)-ergic neurons during development, studying the expression pattern of Ci-GAD (glutamic acid decarboxylase: GABA synthesizing enzyme) by in situ hybridization. Moreover, we cloned two GABA(B) receptor subunits (Ci-GABA(B)Rs), and a phylogenetic analysis (neighbor-joining method) suggested that they clustered with their vertebrate counterparts. We compared Ci-GAD and Ci-GABA(B)Rs expression patterns in C. intestinalis embryos and larvae. At the tailbud stage, Ci-GAD expression was widely detected in central and peripheral nervous system (CNS/PNS) precursors, whereas Ci-GABA(B)Rs expression was evident at the level of the precursors of the visceral ganglion. GABA was localized by immunohistochemistry at the same developmental stage. In the larva, Ci-GAD transcripts and GABA immunofluorescence were also detected throughout the CNS and in some neurons of the PNS, whereas transcripts of both GABA(B) receptor subunits were found mainly in the CNS. The expression pattern of Ci-GABA(B)Rs appeared restricted to Ci-GAD-positive territories in the sensory vesicle, whereas, in the visceral ganglion, Ci-GABA(B)Rs transcripts were found in ventral motoneurons that did not express Ci-GAD. Insofar as GABAergic neurons are widely distributed also in the CNS and PNS of vertebrates and other invertebrate chordates, it seems likely that GABA signaling was extensively present in the protochordate nervous system. Results from this work show that GABA is the most widespread inhibitory neurotransmitter in C. intestinalis nervous system and that it can signal through GABA(B) receptors both pre- and postsynaptically to modulate different sensory inputs and subsequent swimming activity.

Simple chordates exhibit a proton-independent function of acid-sensing ion channels

Acid-sensing ion channels (ASICs) constitute a family of neuron-specific voltage-insensitive sodium channels gated by extracellular protons. Functions of ASICs in mammals include nociception, mechanosensation, and modulation of synaptic transmission. However, the role protons play in mediating the effects of ASICs remains elusive. We have examined ASICs from the ascidian Ciona intestinalis, a simple chordate organism whose nervous system in the larval stage exhibits high similarity to that of higher vertebrates. We found that the ascidian genome contains a single ASIC gene that gives rise to two splice forms analogous to the mammalian ASIC1 and ASIC2. CiASIC is expressed in most neurons of the larva but is absent in the adult. Despite high sequence similarity with mammalian counterparts, CiASIC is proton-insensitive when examined in heterologous systems or in larval neurons; the latter rules out the possibility that proton sensitivity is conferred by accessory proteins or particular factors present only in Ciona neurons. Down-regulation of the CiASIC transcript by double-stranded RNA disrupted the regular pattern of larval swimming, implying that proton-independent mechanisms mediate the effects of ASIC in vivo. Together the data identify ASIC as a highly conserved channel distinctive of chordate nervous systems and show that protons are not essential for ASIC function.

Dynamic change in the expression of developmental genes in the ascidian central nervous system: revisit to the tripartite model and the origin of the midbrain-hindbrain boundary region

Comparative studies on expression patterns of developmental genes along the anterior-posterior axis of the embryonic central nervous system (CNS) between vertebrates and ascidians led to the notion of "tripartite organization," a common ground plan of the CNS, consisting of the anterior, central and posterior regions expressing Otx, Pax2/5/8 and Hox genes, respectively. In ascidians, however, descriptions and interpretations about expression of the developmental genes regarded as region specific have become not necessarily consistent. To address this issue, we examined detailed expression of key developmental genes for the ascidian CNS, including Otx, Pax2/5/8a, En, Fgf8/17/18, Dmbx, Lhx3 and Hox genes, in the CNS around the junction of the trunk and tail of three different tailbud-stage embryos of Ciona intestinalis, employing double-fluorescence in situ hybridization, followed by staining with DAPI to precisely locate expressing cells for each gene. Based on these observations, we have constructed detailed gene expression maps of the region at the tailbud stages. Our analysis shows that expression of several genes regarded as markers for specific domains in the ascidian CNS changes dynamically within a relatively short period. This motivates us to revisit to the tripartite ground plan and the origin of the midbrain-hindbrain boundary (MHB) region.

Primary cultures of nervous system cells from the larva of the ascidian Ciona intestinalis

The ascidian Ciona intestinalis is a useful model for the study of nervous system development and function. The larva of this animal represents a 'primitive' vertebrate form that contains only about 100 neurons in the CNS. Although embryos can be easily subjected to genetic manipulation, the nervous system cells are not easily accessible for neurophysiological study at the larval stage. To remedy this problem, we have developed a method to obtain primary cell cultures from the larval stage of Ciona. Light microscopy and electrophysiology discriminate several types of cells including neurons and photoreceptors. The results show that in Ciona primary cultures different types of neurons as well as neurite sprouting and synapse formation can be visualised. Ciona primary cell cultures will be very useful to study the biochemical, molecular and biophysical properties of individual cells in the larval nervous system of C. intestinalis.

Development of the caudal nerve cord, motoneurons, and muscle innervation in the appendicularian urochordate Oikopleura dioica

The development of the caudal nerve cord and muscle innervation in the appendicularian Oikopleura dioica was assessed using differential interference contrast and confocal microscopy, phalloidin staining of actin, and in situ hybridization for the neuronal markers tubulin and choline acetyltransferase (ChAT). The caudal nerve cord first appears as a stream of tubulin mRNA-positive neurons that extends into the tail from the caudal ganglion. By this stage a few actin-rich nerve fibers course longitudinally along the cord. As the tail lengthens, the caudal nerve cord extends and becomes more fasciculated and the neurons cluster at stereotyped longitudinal positions. The number of neurons in the nerve cord reaches a relatively stable maximum of about 29. A subset of neurons in the caudal ganglion and caudal nerve cord expresses ChAT mRNA. These putative motoneurons are distributed along nearly the full extent of the tail in numbers consistent with an independent innervation of each tail muscle cell. The longitudinal series of putative motoneurons is not aligned with the muscle cells, but peripheral nerve fibers extending to the muscle cells are, indicating that motor axons grow along the cord before exiting adjacent to their peripheral target. Muscle innervation occurs roughly coincident with the onset of ChAT mRNA expression. Our results provide the first molecular identification of motoneurons and the first developmental characterization of the motor system in an appendicularian and help set the stage for gene expression studies aimed at understanding the evolution of developmental patterning in this part of the chordate central nervous system.

Neurons of the ascidian larval nervous system in Ciona intestinalis: I. Central nervous system

The tadpole larva of ascidians, basal living relatives of vertebrates, has a chordate body plan. The CNS has many homologies with that of vertebrates yet only about 100 neurons. These few, possibly fixed in number and composition, nevertheless govern a diverse repertoire of behaviors. To elucidate the circuits of the CNS first requires that we recognize each neuron type, for which we used electroporation to transfect precleavage embryos with a plasmid containing green fluorescent protein (GFP) driven by the promoter of the synaptotagmin gene. Hatched larvae were fixed and GFP 3-D reconstructions of confocal image stacks compiled into images of 31 whole or partial larvae, either with many GFP-labelled neurons or with few, each clearly visible. Neuron counts in the sensory vesicle (SV) and visceral ganglion (VG) indicated that between 75% (SV) and 69% (VG) of previously reported numbers of neurons were transfected. Based on their position, shape, and projections, the following neurons were identified in the SV: a prominent eminens neuron, possibly with direct input from papillar neurons, a large ventroposterior interneuron, photoreceptors of the ocellus, and putative antenna cells of the otolith. In the VG, we identified at least four subtypes of motor neuron, including an ovoid cell that may innervate distal tail muscle cells and contrapelo cells with ascending projections, unique among VG neurons. The caudal nerve cord contained the first reported neurons, the somata of planate neurons. These neurons are the first identified types, and will be used to construct a map of the nervous system for this model basal chordate.

Neurons of the ascidian larval nervous system inCiona intestinalis: II. Peripheral nervous system

The peripheral nervous system of the ascidian tadpole larva comprises a distributed population of isolated receptor neurons, most of unproved function, organized along the trunk or tail epithelium. Previous reports using immunocytochemical methods failed to resolve the detailed morphology of the neurons and their axon pathways. Precleavage embryos of Ciona intestinalis transfected with the promoter of the neuron-specific synaptotagmin gene fused to a green fluorescent protein (GFP) gene yielded clearly labelled GFP profiles. These we examined in confocal image stacks of 31 larvae. Anchor cells, at least eight in each adhesive apical papilla, contribute axons to the papillar nerves that terminate in the sensory vesicle of the central nervous system. Two nerve bundles projected from each papilla, suggesting that at least two subpopulations of papillar neurons exist. Each bundle fasciculated with axons of the rostral trunk epidermal neurons (RTEN) in a stereotyped pattern. The RTEN had a hitherto unreported elaborate arbor of sensory dendrites within the tunic, suggesting that each has an extended sensorial field. Two subpopulations of apical trunk epidermal neurons (ATEN), anterior and posterior, were distinguished. As with the RTEN, these neurons extended dendritic arbors into the tunic. Two additional types of tail neuron, the caudal epidermal neurons (dorsal and ventral) as well as a novel bipolar interneuron, were identified. These identified neuron types are the substrate for the ascidian larva's entire peripheral sensory input, important during larval swimming and settlement.

Ammonium channel expression is essential for brain development and function in the larva ofCiona intestinalis

Ammonium uptake into the cell is known to be mediated by ammonium transport (Amt) proteins, which are present in all domains of life. The physiological role of Amt proteins remains elusive; indeed, loss-of-function experiments suggested that Amt proteins do not play an essential role in bacteria, yeast, and plants. Here we show that the reverse holds true in the tunicate Ciona intestinalis. The genome of C. intestinalis contains two AMT genes, Ci-AMT1a and Ci-AMT1b, which we show derive from an ascidian-specific gene duplication. We analyzed Ci-AMT expression during embryo development. Notably, Ci-AMT1a is expressed in the larval brain in a small number of cells defining a previously unseen V-shaped territory; these cells connect the brain cavity to the external environment. We show that the knockdown of Ci-AMT1a impairs the formation of the brain cavity and consequently the function of the otolith, the gravity-sensing organ contained in it. We speculate that the normal mechanical functioning (flotation and free movement) of the otolith may require a close regulation of ammonium salt(s) concentration in the brain cavity, because ammonium is known to affect both fluid density and viscosity; the cells forming the V territory may act as a conduit in achieving such a regulation.

Development of swimming behaviour in the larva of the ascidian Ciona intestinalis

The aim of this study was to characterize the swimming behaviour of C. intestinalis larvae during the first 6 h after hatching by measuring tail muscle field potentials. This recording method allowed a quantitative description of the responses of the larva under light and dark conditions. Three different larval movements were distinguished by their specific frequencies: tail flicks, 'spontaneous' swimming, and shadow response, or dark induced activity, with respective mean frequencies of about 10, 22 and 32 Hz. The shadow response develops at about 1.5 h post hatching (h.p.h.). The frequency of muscle potentials associated with this behaviour became higher than those of spontaneous swimming activity, shifting from 20 to 30 Hz, but only from about 2 h.p.h. onwards. Swimming rate was influenced positively for about 25 s after the beginning of the shadow response. Comparison of swimming activity at three different larval ages (0-2, 2-4 and 4-6 h.p.h.) showed that Ciona larvae swim for longer periods and more frequently during the first hours after hatching. Our results provide a starting point for future studies that aim to characterize the nervous control of ascidian locomotion, in wild-type or mutant larvae.

The ascidian homolog of the vertebrate homeobox gene Rx is essential for ocellus development and function

The tadpole larvae prosencephalon of the ascidian Ciona intestinalis contains a single large ventricle, along the inner walls of which lie two sensory organs: the otolith (a gravity-sensing organ) and the ocellus (a photo-sensing organ composed of a single cup-shaped pigment cell, about 20 photoreceptor cells, and three lens cells). Comparison has been drawn between the morphology and physiology of photoreceptor cells in the ascidian ocellus and the vertebrate eye. The development of vertebrate and invertebrate eyes requires the activity of several conserved genes and it is regulated by precise expression patterns and cell fate decisions common to several species. We have isolated a Ciona homeobox gene (Ci-Rx) that belongs to the paired-like class of homeobox genes. Rx genes have been identified from a variety of organisms and have been demonstrated to have a role in vertebrate eye formation. Ci-Rx is expressed in the anterior neural plate in the middle tailbud stage and subsequently in the larval stage in the sensory vesicle around the ocellus. Loss of Ci-Rx function leads to an ocellus-less phenotype that shows a loss of photosensitive swimming behavior, suggesting the important role played by Ci-Rx in basal chordate photoreceptor cell differentiation and ocellus formation. Furthermore, studies on Ci-Rx regulatory elements electroporated into Ciona embryos using LacZ or GFP as reporter genes indicate the presence of Ci-Rx in pigment cells, photoreceptors, and neurons surrounding the sensory vesicle. In Ci-Rx knocked-down larvae, neither basal swimming activity nor shadow responses develop. Thus, Rx has a role not only in pigment cells and photoreceptor formation but also in the correct development of the neuronal circuit that controls larval photosensitivity and swimming behavior. The results suggest that a Ci-Rx "retinal" territory exists, which consists of pigment cells, photoreceptors, and neurons involved in transducing the photoreceptor signals.