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Background: Gonadotropin-releasing hormones (GnRHs) are neuropeptides that play central roles in the reproduction of vertebrates. In the ascidian Ciona intestinalis, GnRHs and their receptors are expressed in the nervous systems at the larval stage, when animals are not yet capable of reproduction, suggesting that the hormones have non-reproductive roles. Results: We showed that GnRHs in Ciona are involved in the animal′s metamorphosis by regulating tail absorption and adult organ growth. Absorption of the larval tail and growth of the adult organs are two major events in the metamorphosis of ascidians. When larvae were treated with GnRHs, they completed tail absorption more frequently than control larvae. cAMP was suggested to be a second messenger for the induction of tail absorption by GnRHs. tGnRH-3 and tGnRH-5 (the 't' indicates 'tunicate') inhibited the growth of adult organs by arresting cell cycle progression in parallel with the promotion of tail absorption. Conclusions: This study provides new insights into the molecular mechanisms of ascidian metamorphosis conducted by non-reproductive GnRHs. Developmental Dynamics 243:1524-1535, 2014. © 2014 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The tunicate Ciona intestinalis larva has a simple central nervous system (CNS), consisting of fewer than 400 cells, which is homologous to the vertebrate CNS. Recent studies have revealed neuronal types and networks in the larval CNS of C. intestinalis, yet their cell lineage and the molecular mechanism by which particular types of neurons are specified and differentiate remain poorly understood. Here, we report cell lineage origin and a cis-regulatory module for the anterior caudal inhibitory neurons (ACINs), a putative component of the central pattern generator regulating swimming locomotion. The vesicular GABA/glycine transporter gene Ci-VGAT, a specific marker for GABAergic/glycinergic neurons, is expressed in distinct sets of neurons, including ACINs of the tail nerve cord and others in the brain vesicle and motor ganglion. Comparative genomics analysis between C. intestinalis and Ciona savignyi and functional analysis in vivo identified the cis-regulatory module responsible for Ci-VGAT expression in ACINs. Our cell lineage analyses inferred that ACINs derive from A11.116 cells, which have been thought to solely give rise to glial ependymal cells of the lateral wall of the nerve cord. The present findings will provide a solid basis for future studies addressing the molecular mechanism underlying specification of ACINs, which play a critical role in controlling larval locomotion. [ABSTRACT FROM AUTHOR]

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, γ-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. [ABSTRACT FROM AUTHOR]

The larval nervous system of the ascidian Ciona intestinalis exhibits an abstract form of the vertebrate nervous system. The Ci-Gαi1 gene, which encodes a G-protein α subunit, is specifically expressed in distinct sets of neurons in C. intestinalis larvae, including papillar neurons of the adhesive organ, ocellus photoreceptor cells, and cholinergic and GABAergic neurons in the central nervous system (CNS). A GFP reporter gene driven by the 4.2-kb 5′ flanking region of Ci-Gαi1 recapitulated the endogenous gene expression patterns. Comparative genomic analysis of the Gαi1 gene between C. intestinalis and Ciona savignyi identified an 87-bp highly conserved non-coding sequence located between –3176 and –3090 bp upstream of the gene. Deletion of this conserved upstream sequence resulted in the complete loss of reporter expression in the central nervous system, while reporter expression in the adhesive organ and mesenchyme cells remained unaffected. The conserved upstream sequence can activate gene expression from basal promoters in the brain vesicle, although it requires additional cis-regulatory sequences to fully activate the CNS-specific gene expression. These results suggest that different types of central neurons share a common transcriptional activation mechanism that is different from that of papillar neurons. [ABSTRACT FROM AUTHOR]

The brain vesicle of the tadpole larva of ascidians, simple basal chordates, contains an eye-spot (ocellus), which is responsible for the photic swimming behavior. Ascidian adults also exhibit several types of light-responsive behaviors. Molecular phylogenetic studies have suggested that ascidians are the closest living relatives of vertebrates, and therefore, understanding the photoreceptive systems in ascidians is a key to uncover the origin and evolution of the vertebrate eyes. The ocellus of the ascidian larva has ciliary photoreceptors resembling those of the retina and pineal eye of vertebrates. Recent studies have indicated that the ascidian larva has phototransduction and visual cycle systems similar to those of vertebrate eyes. Comparative studies on photoreceptor systems between ascidians and vertebrates provide us clues to reconstructing the evolutionary pathway leading to the lateral and median eyes of vertebrates. [ABSTRACT FROM AUTHOR]

In vertebrates, the absorption of light by rhodopsin leads to the isomerization of 11- cis-retinal chromophore to its all- trans form. In the visual cycle, all- trans retinal is converted back to 11- cis retinal. Mammalian visual cycle takes place in photoreceptor cells and retinal pigment epithelial (RPE) cells, while that of cephalopods is completed within a photoreceptor cell. To identify visual cycle system in the primitive chordate ascidians, we studied the localization of the ascidian visual cycle genes and proteins by in situ hybridization and whole-mount immunohistochemistry, respectively. We identified four genes encoding putative visual cycle proteins, homologs of retinal G protein-coupled receptor ( Ci-opsin3), cellular retinaldehyde-binding protein ( Ci-CRALBP), β-carotene 15,15′monooxygenase ( Ci-BCO) and RPE-specific 65 kDa protein ( Ci-RPE65) in the ascidian, Ciona intestinalis. In contrast to Ci-BCO, which is predominantly localized in ocellus photoreceptor cells of the larva, Ci-RPE65 is not significantly expressed in the ocellus and brain vesicle of the larva. Ci-RPE65 is expressed in the neural complex, a photoreceptor organ of the adult ascidian, at a level comparable with that of Ci-opsin3 and Ci-CRALBP. Proteins of Ci-opsin3, Ci-CRALBP and Ci-BCO are localized in photoreceptor cells. These results suggest that the larval visual cycle uses Ci-opsin3 as a photoisomerase, while the visual cycle of the adult photoreceptors is RPE65-dependent. The colocalization of visual cycle proteins in the photoreceptor cells suggest that ascidian visual cycle takes place in a photoreceptor cell as seen in the cephalopod visual cycle. [ABSTRACT FROM AUTHOR]

Two cDNA clones encoding α and β subunits of soluble guanylyl cyclase (GC) were isolated from medaka fish (Oryzias latipes) cDNA libraries and designated as OlGCS-α1 and OlGCS-β1, respectively. The ORF of OlGCS-α1 encodes 678 amino acids and that of OlGCS-β1 encodes 614 amino acids. Both subunits have a putative catalytic domain conserved with membrane GC and adenylyl cyclases. The amino acid sequences of OlGCS-β1 and mammalian soluble GC subunits are highly conserved throughout the entire protein, suggesting that amino acid sequences outside the catalytic domain are important for the functioning of the β subunit. On the contrary, amino acid sequences of α subunits outside the catalytic domain are fairly divergent between fish and mammals. Reverse transcription/PCR (RT/PCR) analysis showed that OlGCS-α1 and OlGCS-β1 transcripts were abundant in the brain, eye, spleen and testis. RT/PCR analysis demonstrated that both transcripts were present in unfertilized eggs, were reduced immediately after fertilization, then increased again. In addition, our results suggest that expression patterns of OlGCS-α1 and OlGCS-β1 genes are different from each other both during development and in adults. [ABSTRACT FROM AUTHOR]