ABSTRACT
Gonadotropin-releasing hormones (GnRHs) play pivotal roles in reproduction via the hypothalamus-pituitary-gonad axis (HPG axis) in vertebrates. GnRHs and their receptors (GnRHRs) are also conserved in invertebrates lacking the HPG axis, indicating that invertebrate GnRHs do not serve as "gonadotropin-releasing factors" but, rather, function as neuropeptides that directly regulate target tissues. All vertebrate and urochordate GnRHs comprise 10 amino acids, whereas amphioxus, echinoderm, and protostome GnRH-like peptides are 11- or 12-residue peptides. Intracellular calcium mobilization is the major second messenger for GnRH signaling in cephalochordates, echinoderms, and protostomes, while urochordate GnRHRs also stimulate cAMP production pathways. Moreover, the ligand-specific modulation of signal transduction via heterodimerization between GnRHR paralogs indicates species-specific evolution in Ciona intestinalis. The characterization of authentic or putative invertebrate GnRHRs in various tissues and their in vitro and in vivo activities indicate that invertebrate GnRHs are responsible for the regulation of both reproductive and nonreproductive functions. In this review, we examine our current understanding of and perspectives on the primary sequences, tissue distribution of mRNA expression, signal transduction, and biological functions of invertebrate GnRHs and their receptors.
Subject(s)
Hypothalamus/metabolism , Invertebrates/metabolism , Receptors, LHRH/metabolism , Animals , Biological Evolution , COS Cells , Calcium/metabolism , Chlorocebus aethiops , Ciona intestinalis , Cyclic AMP/metabolism , Echinodermata , Female , Gonadotropin-Releasing Hormone/metabolism , HEK293 Cells , Humans , Ligands , Male , Markov Chains , Mollusca , Signal Transduction , Tissue Distribution , UrochordataABSTRACT
Despite containing only approximately 330 cells, the central nervous system (CNS) of Ciona intestinalis larvae has an architecture that is similar to the vertebrate CNS. Although only vertebrates have a distinct hypothalamus-the source of numerous neurohormone peptides that play pivotal roles in the development, function, and maintenance of various neuronal and endocrine systems, it is suggested that the Ciona brain contains a region that corresponds to the vertebrate hypothalamus. To identify genes expressed in the brain, we isolated brain vesicles using transgenic embryos carrying Ci-ß-tubulin(promoter)::Kaede, which resulted in robust Kaede expression in the larval CNS. The associated transcriptome was investigated using microarray analysis. We identified 565 genes that were preferentially expressed in the larval brain. Among these genes, 11 encoded neurohormone peptides including such hypothalamic peptides as gonadotropin-releasing hormone and oxytocin/vasopressin. Six of the identified peptide genes had not been previously described. We also found that genes encoding receptors for some of the peptides were expressed in the brain. Interestingly, whole-mount in situ hybridization showed that most of the peptide genes were expressed in the ventral brain. This catalog of the genes expressed in the larval brain should help elucidate the evolution, development, and functioning of the chordate brain.
Subject(s)
Brain/growth & development , Brain/metabolism , Ciona intestinalis/growth & development , Ciona intestinalis/genetics , Amino Acid Sequence , Animals , Animals, Genetically Modified , Ciona intestinalis/metabolism , Evolution, Molecular , Gene Expression Regulation, Developmental , Hypothalamus/growth & development , Hypothalamus/metabolism , In Situ Hybridization , Invertebrate Hormones/genetics , Larva/growth & development , Larva/metabolism , Molecular Sequence Data , Neuropeptides/genetics , Oligonucleotide Array Sequence Analysis , Receptors, G-Protein-Coupled/genetics , Receptors, Neuropeptide/genetics , Reverse Transcriptase Polymerase Chain Reaction , Signal Transduction/genetics , Transcription Factors/genetics , Vertebrates/growth & development , Vertebrates/metabolismABSTRACT
Annetocin is structurally related to an OT (oxytocin)/VP (vasopressin) family peptide, which has been isolated from the earthworm Eisenia foetida and has been shown to induce OT-like egg-laying behaviour. We now report the identification of an endogenous AnR (annetocin receptor). The deduced AnR precursor displays high sequence similarity with OT/VP receptors. Genomic analysis of the AnR gene revealed that the intron-inserted position is conserved between the AnR gene and the mammalian OT/VP receptor genes. These results indicate that AnR and mammalian OT/VP receptors share a common ancestor gene. Administration of annetocin to the AnR expressed in Xenopus oocytes induced a calcium-dependent signal transduction. Reverse transcriptase-PCR analysis and in situ hybridization showed that the AnR gene is expressed specifically in the nephridia located in the clitellum region, although the nephridia are distributed throughout the worm body. This result suggests that annetocin induces egg-laying behaviour through its action on the nephridia. This is the first description concerning the functional correlation between an invertebrate OT/VP-related peptide and egg-laying behaviour.
Subject(s)
Oxytocin/metabolism , Receptors, G-Protein-Coupled/genetics , Receptors, Invertebrate Peptide/genetics , Vasopressins/metabolism , Amino Acid Sequence , Animals , Cloning, Molecular/methods , DNA, Complementary/genetics , Exons/genetics , Gene Expression Regulation/physiology , Gene Transfer Techniques , Introns/genetics , Molecular Sequence Data , Oligochaeta/anatomy & histology , Oligochaeta/chemistry , Oligochaeta/cytology , Oligochaeta/genetics , Oocytes/chemistry , Oocytes/metabolism , Open Reading Frames/genetics , Pituitary Hormones, Posterior , RNA, Messenger/metabolism , Receptors, G-Protein-Coupled/chemistry , Receptors, G-Protein-Coupled/physiology , Receptors, Invertebrate Peptide/chemistry , Receptors, Invertebrate Peptide/physiology , Xenopus laevis/geneticsABSTRACT
Structurally tachykinin-related peptides have been isolated from various invertebrate species and shown to exhibit their biological activities through a G-protein-coupled receptor (GPCR) for a tachykinin-related peptide. In this paper, we report the identification of a novel tachykinin-related peptide receptor, the urechistachykinin receptor (UTKR) from the echiuroid worm, Urechis unitinctus. The deduced UTKR precursor includes seven transmembrane domains and typical sites for mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors. A functional analysis of the UTKR expressed in Xenopus oocytes demonstrated that UTKR, like tachykinin receptors and tachykinin-related peptide receptors, activates calcium-dependent signal transduction upon binding to its endogenous ligands, urechistachykinins (Uru-TKs) I-V and VII, which were isolated as Urechis tachykinin-related peptides from the nervous tissue of the Urechis unitinctus in our previous study. UTKR responded to all Uru-TKs equivalently, showing that UTKR possesses no selective affinity with Uru-TKs. In contrast, UTKR was not activated by substance P or an Uru-TK analog containing a C-terminal Met-NH2 instead of Arg-NH2. Furthermore, the genomic analysis revealed that the UTKR gene, like mammalian tachykinin receptor genes, consists of five exons interrupted by four introns, and all the intron-inserted positions are completely compatible with those of mammalian tachykinin receptor genes. These results suggest that mammalian tachykinin receptors and invertebrate tachykinin-related peptide receptors were evolved from a common ancestral GPCR gene. This is the first identification of an invertebrate tachykinin-related peptide receptor from other species than insects and also of the genomic structure of a tachykinin-related peptide receptor gene.