Neuronal organization of the ascidian (Urochordata) branchial basket revealed by cholinesterase activity

Two distinct evolutionary conserved neural degeneration pathways characterized in a colonial chordate

Colonial tunicates are marine organisms that possess multiple brains simultaneously during their colonial phase. While the cyclical processes of neurogenesis and neurodegeneration characterizing their life cycle have been documented previously, the cellular and molecular changes associated with such processes and their relationship with variation in brain morphology and individual (zooid) behavior throughout adult life remains unknown. Here, we introduce Botryllus schlosseri as an invertebrate model for neurogenesis, neural degeneration, and evolutionary neuroscience. Our analysis reveals that during the weekly colony budding (i.e., asexual reproduction), prior to programmed cell death and removal by phagocytes, decreases in the number of neurons in the adult brain are associated with reduced behavioral response and significant change in the expression of 73 mammalian homologous genes associated with neurodegenerative disease. Similarly, when comparing young colonies (1 to 2 y of age) to those reared in a laboratory for ∼20 y, we found that older colonies contained significantly fewer neurons and exhibited reduced behavioral response alongside changes in the expression of 148 such genes (35 of which were differentially expressed across both timescales). The existence of two distinct yet apparently related neurodegenerative pathways represents a novel platform to study the gene products governing the relationship between aging, neural regeneration and degeneration, and loss of nervous system function. Indeed, as a member of an evolutionary clade considered to be a sister group of vertebrates, this organism may be a fundamental resource in understanding how evolution has shaped these processes across phylogeny and obtaining mechanistic insight.

An α7-related nicotinic acetylcholine receptor mediates the ciliary arrest response in pharyngeal gill slits of Ciona

Ciliary movement is a fundamental process to support animal life, and the movement pattern may be altered in response to external stimuli under the control of nervous systems. Juvenile and adult ascidians have ciliary arrays around their pharyngeal gill slits (stigmata), and continuous beating is interrupted for seconds by mechanical stimuli on other parts of the body. Although it has been suggested that neural transmission to evoke ciliary arrest is cholinergic, its molecular basis has not yet been elucidated in detail. Here, we attempted to clarify the molecular mechanisms underlying this neurociliary transmission in the model ascidian Ciona Acetylcholinesterase histochemical staining showed strong signals on the laterodistal ciliated cells of stigmata, hereafter referred to as trapezial cells. The direct administration of acetylcholine (ACh) and other agonists of nicotinic ACh receptors (nAChRs) onto ciliated cells reliably evoked ciliary arrest that persisted for seconds in a dose-dependent manner. While the Ciona genome encodes ten nAChRs, only one of these called nAChR-A7/8-1, a relative of vertebrate α7 nAChRs, was found to be expressed by trapezial cells. Exogenously expressed nAChR-A7/8-1 on Xenopus oocytes responded to ACh and other agonists with consistent pharmacological traits to those observed in vivo Further efforts to examine signaling downstream of this receptor revealed that an inhibitor of phospholipase C (PLC) hampered ACh-induced ciliary arrest. We propose that homomeric α7-related nAChR-A7/8-1 mediates neurociliary transmission in Ciona stigmata to elicit persistent ciliary arrest by recruiting intracellular Ca2+ signaling.

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.

Hair cells in an ascidian (Tunicata) and their evolution in chordates

In ascidians, mechanoreceptors of the oral area are involved in monitoring the incoming water flow. Sensory cells are represented by scattered, ciliated primary cells (sending their own axons to the cerebral ganglion) or secondary sensory cells (axonless cells forming afferent and efferent synapses with neurons, whose somata are located in the ganglion) of the coronal organ. Coronal cells have varying morphologies: in species of the Enterogona order, they are multiciliate, whereas those of Pleurogona possess an apical apparatus composed of one or two cilia accompanied by stereovilli, in some cases also graded in length. The coronal organ has been proposed as a homologue to the vertebrate octavo-lateralis system, because coronal cells resemble vertebrate hair cells for morphology, embryonic origin and arrangement. In the ascidian Molgula socialis (Pleurogona), we now describe the morphology of the coronal organ, which contains a few associated rows of sensory cells that run the whole length of the oral velum and the branched tentacles. Three kinds of sensory cells, accompanied by specialised supporting cells, are present. Comparisons between the coronal organ and other chordate mechanosensory structures suggest that hair cells originated in the common ancestor of chordates.

Morphological and gene expression similarities suggest that the ascidian neural gland may be osmoregulatory and homologous to vertebrate peri-ventricular organs

Summary The central nervous system (cerebral ganglion) of adult ascidians is linked to the neural gland complex (NGC), which consists of a dorsal tubercle, a ciliated duct and a neural gland. The function of the NGC has been the subject of much debate. The recent publication of the complete genomic sequence of Ciona intestinalis provides new opportunities to examine the presence and distribution of protein families in this basal chordate. We focus here on the ascidian neuropeptide G-protein-coupled receptors (GPCRs), the vertebrate homologues of which are involved in homeostasis. In situ hybridization revealed that five Ciona GPCRs [vasopressin receptor, somatostatin receptor, CRH (corticotropin-releasing hormone) receptor, angiotensin receptor and tachykinin receptor] are expressed in the NGC of adult ascidians. These findings, together with histological and ultrastructural data, provide evidence to support a role for the ascidian NGC in maintaining ionic homeostasis. We further speculate about the potential similarities between the ascidian NGC and the vertebrate choroid plexus, a neural peri-ventricular organ.

The nervous system in adult tunicates: current research directions

This review covers 25 years of progress on structural, functional, and developmental neurobiology of adult tunicates. The focus is on ascidians rather than pelagic species. The ascidian brain and peripheral nervous system are considered from the point of view of ultrastructure, neurotransmitters, regulatory peptides, and electrical activity. Sensory reception and effector control are stressed. Discussion of the dorsal strand plexus centres on its relationship with photoreceptors, the presence in it of gonadotropin-releasing hormone and its role in reproductive control. In addition to hydrodynamic sense organs based on primary sensory neurons (cupular organs), ascidians are now known to have coronal sense organs based on axonless hair cells resembling those of the vertebrate acustico-lateralis system. The peripheral nervous system is remarkable in that the motor neuron terminals are apparently interconnected synaptically, providing the equivalent of a nerve net. Development of the neural complex in ascidians is reviewed, highlighting recent embryological and molecular evidence for stomodeal, neurohypophyseal, and atrial placodes. The nervous system forms similarly during embryogenesis in the oozooid and blastogenesis in colonial forms. The regeneration of the brain in Ciona intestinalis (L., 1767) is discussed in relation to normal neurogenesis. Finally, the viviparous development of salps is considered, where recent work traces the early development of the brain, outgrowth of nerve roots, and the targetting of motor nerves to the appropriate muscles.

Endocrinology of protochordates

Large-scale gene duplications occurred early in the vertebrate lineage after the split with protochordates. Thus, protochordate hormones and their receptors, transcription factors, and signaling pathways may be the foundation for the endocrine system in vertebrates. A number of hormones have been identified including cionin, a likely ancestor of cholecytokinin (CCK) and gastrin. Both insulin and insulin-like growth hormone (IGF) have been identified in separate cDNAs in a tunicate, whereas only a single insulin-like peptide was found in amphioxus. In tunicates, nine distinct forms of gonadotropin-releasing hormone (GnRH) are shown to induce gamete release, even though a pituitary gland and sex steroids are lacking. In both tunicates and amphioxus, there is evidence of some components of a thyroid system, but the lack of a sequenced genome for amphioxus has slowed progress in the structural identification of its hormones. Immunocytochemistry has been used to tentatively identify a number of hormones in protochordates, but structural and functional studies are needed. For receptors, protochordates have many vertebrate homologs of nuclear receptors, such as the thyroid, retinoic acid, and retinoid X receptors. Also, tunicates have cell surface receptors including the G-protein-coupled type, such as β-adrenergic, putative endocannabinoid, cionin (CCK-like), and two GnRH receptors. Several tyrosine kinase receptors include two epidermal growth factor (EGF) receptors (tunicates) and an insulin/IGF receptor (amphioxus). Interestingly, neither steroid receptors nor a full complement of enzymes for synthesis of sex steroids are encoded in the Ciona genome. Tunicates appear to have some but not all of the necessary molecules to develop a vertebrate-like pituitary or complete thyroid system.

Novel, secondary sensory cell organ in ascidians: in search of the ancestor of the vertebrate lateral line

A new mechanoreceptor organ, the "coronal organ," located in the oral siphon, is described by light and electron microscopy in the colonial ascidians Botryllus schlosseri and Botrylloides violaceus. It is composed of a line of sensory cells (hair cells), accompanied by supporting cells, that runs continuously along the margin of the velum and tentacles of the siphon. These hair cells resemble those of the vertebrate lateral line or, in general, the acoustico-lateralis system, because they bear a single cilium, located centrally or eccentrically to a hair bundle of numerous stereovilli. In contrast to other sensory cells of ascidians, the coronal hair cells are secondary sensory cells, since they lack axonal processes directed towards the cerebral ganglion. Moreover, at their base they form synapses with nerve fibers, most of which exhibit acetylcholinesterase activity. The absence of axonal extensions was confirmed by experiments with lipophilic dyes. Different kinds of synapses were recognized: usually, each hair cell forms a few afferent synapses with dendrites of neurons located in the ganglion; efferent synapses, both axo-somatic (between an axon coming from the ganglion and the hair cell) and axo-dendritic (between an axon coming from the ganglion and an afferent fiber) were occasionally found. The presence of secondary sensory cells in ascidians is discussed in relation to the evolution of sensory cells and placodes in vertebrates. It is proposed that the coronal organ in urochordates is homologous to the vertebrate acoustico-lateralis system.

Development of the motor nervous system in ascidians

The motor nervous system of adult ascidians consists of neurons forming the cerebral ganglion from which axons run out directly to the effectors, i.e., muscular and ciliary cells. In this study, we analyzed the development of the motor fibers, correlating this with organ differentiation during asexual reproduction in Botryllus schlosseri. We used a staining method for acetylcholinesterase, whose reaction product is visible with both light and electron microscopy and which labels entire nerves, including their thin terminals, making them identifiable between tissues. While the cerebral ganglion is forming, the axons elongate and follow stereotypical pathways to reach the smooth muscle cells of the body, the striated muscle of the heart, and the ciliated cells of the branchial stigmata and the gut. A strict temporal relation links the development of the local neural network with its target organ, which is approached by nerves before the effector cells are fully differentiated. This process occurs for oral and cloacal siphons, branchial basket, gut, and heart. Axons grow through the extracellular matrix and arrive at their targets from different directions. In some cases, the blood sinuses constitute the favorite roads for growing axons, which seem to be guided by a mechanism involving contact guidance or stereotropism. The pattern of innervation undergoes dynamic rearrangements and a marked process of elimination of axons, when the last stages of blastogenesis occur. The final pattern of motor innervation seems to be regulated by axon withdrawal, rather than apoptosis of motor neurons.

Induction of gamete release by gonadotropin-releasing hormone in a protochordate, Ciona intestinalis

Gonadotropin-releasing hormone (GnRH) of vertebrates is now believed to have multiple functions in addition to its role as a hypophysiotropic hormone, as originally defined. Recently, it has been shown that GnRH occurs also in the ascidians, which are considered ancestral chordates. Here the author shows that GnRH induces spawning of gametes from mature individuals of Ciona intestinalis. Ciona accumulates mature gametes in the gonoducts and maintains them until spawning is triggered by a photoperiodic cue(s). Injection of synthetic tunicate GnRH-I or -II into various sites of mature individuals effectively induced gamete release (spawning), although the former was more potent. Gamete release often occurred on a larger scale than in spontaneous spawning. However, moderate gamete release, similar to spontaneous spawning, was often triggered by exogenous tunicate GnRH. GnRH in vivo apparently is released from the GnRH-containing neurons that are distributed from the region of the cerebral ganglion to the proximal part of the ovary along the dorsal strand within the blood sinus; this indicates that both forms of tunicate GnRH may be the actual inducers of spawning. It is suggested that, in the ancestral chordate, GnRH neurons release GnRH prior to the spawning and the released GnRH acts directly on the epithelium of gonoducts or functions as a neuromodulator of other neurons innervating the gonoducts to induce spawning.

Conduction and coordination in deganglionated ascidians

The behaviour of Chelyosoma productum and Corella inflata (Ascidiacea) was studied in normal and deganglionated animals. Chelyosoma productum lived for over a year after deganglionation and the ganglion did not regenerate. Electrophysiological recordings were made from semi-intact preparations. Responses to stimulation and spontaneous activity continued to be transmitted through the body wall and branchial sac after deganglionation. Spread was slow, decremental, and facilitative. Treatment with >10 µg·mL-1 d-tubocurarine abolished all responses, indicating that nerves mediate conduction of excitation after deganglionation. Histological study using cholinesterase histochemistry and immunolabelling with antisera against tubulin and gonadotropin-releasing hormone showed no evidence of a peri pheral nerve net in regions showing conduction, contrary to previous claims. The cell bodies of the motor neurones were found to lie entirely within the ganglion or its major roots. Their terminal branches intermingled to form netlike arrays. Sensory neurons were identified with cell bodies in the periphery, in both the body wall and the branchial sac. Their processes also intermingled in netlike arrays before entering nerves going to the ganglion. It is concluded that the "residual" innervation that survives deganglionation is composed of either interconnected motor nerve terminals, interconnected sensory neurites, or some combination of the two. In re-inventing the nerve net, ascidians show convergent evolution with sea anemones, possibly as an adaptation to a sessile existence.

Ciliary reversal and locomotory control in the pluteus larva of Lytechinus pictus

Normal swimming behaviour of Lytechinus pictus larvae and the effects of selected drugs are described, based on direct observation and videotapes of free-swimming and tethered larvae. The principal effector response is a coordinated ciliary reversal that enables larvae to back away from obstacles and avoid entanglement. The effect is best seen in the epaulettes, whose large size makes the pluteus an especially favourable subject for behavioural observation and tests. Reversals and sustained arrests can be induced by various cholinergic agonists, notably nicotine, which is active to concentrations of 0.2 μM. Dopamine and adrenaline cause reversals and arrests as well, but they act initially on the response as a whole, increasing its frequency, rather than directly on ciliary beat. The data suggest a two-step control sequence, with an initial catecholamine-dependent step that triggers a cholinergic effector response.