Identification and characterization of a novel amphioxus dopamine D-like receptor

Aldosterone, STX and amyloid-β1-42 peptides modulate GPER (GPR30) signalling in an embryonic mouse hippocampal cell line (mHippoE-18)

The GPCR, GPER, mediates many of the rapid, non-genomic actions of 17β-estradiol in multiple tissues, including the nervous system. Controversially, it has also been suggested to be activated by aldosterone, and by the non-steroidal diphenylacrylamide compound, STX, in some preparations. Here, the ability of the GPER agonist, G-1, and aldosterone in the presence of the mineralocorticoid receptor antagonist, eplerenone, to potentiate forskolin-stimulated cyclic AMP levels in the hippocampal clonal cell line, mHippoE-18, are compared. Both stimulatory effects are blocked by the GPER antagonist G36, by PTX, (suggesting the involvement of Gi/o G proteins), by BAPTA-AM, (suggesting they are calcium sensitive), by wortmannin (suggesting an involvement of PI3Kinase) and by soluble amyloid-β peptides. STX also stimulates cyclic AMP levels in mHippoE-18 cells and these effects are blocked by G36 and PTX, as well as by amyloid-β peptides. This suggests that both aldosterone and STX may modulate GPER signalling in mHippoE-18 cells.

Evolution of dopamine receptors: phylogenetic evidence suggests a later origin of the DRD2l and DRD4rs dopamine receptor gene lineages

Dopamine receptors are integral membrane proteins whose endogenous ligand is dopamine. They play a fundamental role in the central nervous system and dysfunction of dopaminergic neurotransmission is responsible for the generation of a variety of neuropsychiatric disorders. From an evolutionary standpoint, phylogenetic relationships among the DRD₁ class of dopamine receptors are still a matter of debate as in the literature different tree topologies have been proposed. In contrast, phylogenetic relationships among the DRD₂ group of receptors are well understood. Understanding the time of origin of the different dopamine receptors is also an issue that needs further study, especially for the genes that have restricted phyletic distributions (e.g., DRD_2l and DRD_4rs). Thus, the goal of this study was to investigate the evolution of dopamine receptors, with emphasis on shedding light on the phylogenetic relationships among the D₁ class of dopamine receptors and the time of origin of the DRD_2l and DRD_4rs gene lineages. Our results recovered the monophyly of the two groups of dopamine receptors. Within the DRD₁ group the monophyly of each paralog was recovered with strong support, and phylogenetic relationships among them were well resolved. Within the DRD₁ class of dopamine receptors we recovered the sister group relationship between the DRD_1C and DRD_1E, and this clade was recovered sister to a cyclostome sequence. The DRD₁ clade was recovered sister to the aforementioned clade, and the group containing DRD₅ receptors was sister to all other DRD₁ paralogs. In agreement with the literature, among the DRD₂ class of receptors, DRD₂ was recovered sister to DRD₃, whereas DRD₄ was sister to the DRD₂/DRD₃ clade. According to our phylogenetic tree, the DRD_2l and DRD_4rs gene lineages would have originated in the ancestor of gnathostomes between 615 and 473 mya. Conservation of sequences required for dopaminergic neurotransmission and small changes in regulatory regions suggest a functional refinement of the dopaminergic pathways along evolution.

Characterisation of Signalling by the Endogenous GPER1 (GPR30) Receptor in an Embryonic Mouse Hippocampal Cell Line (mHippoE-18)

Estrogen can modulate neuronal development and signalling by both genomic and non-genomic pathways. Many of its rapid, non-genomic effects on nervous tissue have been suggested to be mediated via the activation of the estrogen sensitive G-protein coupled receptor (GPER1 or GPR30). There has been much controversy over the cellular location, signalling properties and endogenous activators of GPER1. Here we describe the pharmacology and signalling properties of GPER1 in an immortalized embryonic hippocampal cell line, mHippoE-18. This cell line does not suffer from the inherent problems associated with the study of this receptor in native tissue or the problems associated with heterologously expression in clonal cell lines. In mHippoE-18 cells, 17β-Estradiol can mediate a dose-dependent rapid potentiation of forskolin-stimulated cyclic AMP levels but does not appear to activate the ERK1/2 pathway. The effect of 17β-Estradiol can be mimicked by the GPER1 agonist, G1, and also by tamoxifen and ICI 182,780 which activate GPER1 in a variety of other preparations. The response is not mimicked by the application of the classical estrogen receptor agonists, PPT, (an ERα agonist) or DPN, (an ERβ agonist), further suggesting that this effect of 17β-Estradiol is mediated through the activation of GPER1. However, after exposure of the cells to the GPER1 specific antagonists, G15 and G36, the stimulatory effects of the above agonists are replaced by dose-dependent inhibitions of forskolin-stimulated cyclic AMP levels. This inhibitory effect is mimicked by aldosterone in a dose-dependent way even in the absence of the GPER1 antagonists. The results are discussed in terms of possible "Biased Antagonism" whereby the antagonists change the conformation of the receptor resulting in changes in the agonist induced coupling of the receptor to different second messenger pathways.

Classification of Dopamine Receptor Genes in Vertebrates: Nine Subtypes in Osteichthyes

Dopamine neurotransmission regulates various brain functions, and its regulatory roles are mediated by two families of G protein-coupled receptors: the D1 and D2 receptor families. In mammals, the D1 family comprises two receptor subtypes (D1 and D5), while the D2 family comprises three receptor subtypes (D2, D3 and D4). Phylogenetic analyses of dopamine receptor genes strongly suggest that the common ancestor of Osteichthyes (bony jawed vertebrates) possessed four subtypes in the D1 family and five subtypes in the D2 family. Mammals have secondarily lost almost half of the ancestral dopamine receptor genes, whereas nonmammalian species kept many of them. Although the mammalian situation is an exception among Osteichthyes, the current classification and characterization of dopamine receptors are based on mammalian features, which have led to confusion in the identification of dopamine receptor subtypes in nonmammalian species. Here we begin by reviewing the history of the discovery of dopamine receptors in vertebrates. The recent genome sequencing of coelacanth, gar and elephant shark led to the proposal of a refined scenario of evolution of dopamine receptor genes. We also discuss a current problem of nomenclature of dopamine receptors. Following the official nomenclature of mammalian dopamine receptors from D1 to D5, we propose to name newly identified receptor subtypes from D6 to D9 in order to facilitate the use of an identical name for orthologous genes among different species. To promote a nomenclature change which allows distinguishing the two dopamine receptor families, a nomenclature consortium is needed. This comparative perspective is crucial to correctly interpret data obtained in animal studies on dopamine-related brain disorders, and more fundamentally, to understand the characteristics of dopamine neurotransmission in vertebrates.

Characterisation of AmphiAmR11, an amphioxus (Branchiostoma floridae) D2-dopamine-like G protein-coupled receptor

The evolution of the biogenic amine signalling system in vertebrates is unclear. However, insights can be obtained from studying the structures and signalling properties of biogenic amine receptors from the protochordate, amphioxus, which is an invertebrate species that exists at the base of the chordate lineage. Here we describe the signalling properties of AmphiAmR11, an amphioxus (Branchiostoma floridae) G protein-coupled receptor which has structural similarities to vertebrate α₂-adrenergic receptors but which functionally acts as a D₂ dopamine-like receptor when expressed in Chinese hamster ovary -K1 cells. AmphiAmR11 inhibits forskolin-stimulated cyclic AMP levels with tyramine, phenylethylamine and dopamine being the most potent agonists. AmphiAmR11 also increases mitogen-activated protein kinase activity and calcium mobilisation, and in both pathways, dopamine was found to be more potent than tyramine. Thus, differences in the relative effectiveness of various agonists in the different second messenger assay systems suggest that the receptor displays agonist-specific coupling (biased agonism) whereby different agonists stabilize different conformations of the receptor which lead to the enhancement of one signalling pathway over another. The present study provides insights into the evolution of α₂-adrenergic receptor signalling and support the hypothesis that α₂-adrenergic receptors evolved from D₂-dopamine receptors. The AmphiAmR11 receptor may represent a transition state between D₂-dopamine receptors and α₂-adrenergic receptors.

A comparison of the signalling properties of two tyramine receptors from Drosophila

In invertebrates, the phenolamines, tyramine and octopamine, mediate many functional roles usually associated with the catecholamines, noradrenaline and adrenaline, in vertebrates. The α- and β-adrenergic classes of insect octopamine receptor are better activated by octopamine than tyramine. Similarly, the Tyramine 1 subgroup of receptors (or Octopamine/Tyramine receptors) are better activated by tyramine than octopamine. However, recently, a new Tyramine 2 subgroup of receptors was identified, which appears to be activated highly preferentially by tyramine. We examined immunocytochemically the ability of CG7431, the founding member of this subgroup from Drosophila melanogaster, to be internalized in transfected Chinese hamster ovary (CHO) cells by different agonists. It was only internalized after activation by tyramine. Conversely, the structurally related receptor, CG16766, was internalized by a number of biogenic amines, including octopamine, dopamine, noradrenaline, adrenaline, which also were able to elevate cyclic AMP levels. Studies with synthetic agonists and antagonists confirm that CG16766 has a different pharmacological profile to that of CG7431. Species orthologues of CG16766 were only found in Drosophila species, whereas orthologues of CG7431 could be identified in the genomes of a number of insect species. We propose that CG16766 represents a new group of tyramine receptors, which we have designated the Tyramine 3 receptors.

Evolution of dopamine receptor genes of the D1 class in vertebrates

The receptors of the dopamine neurotransmitter belong to two unrelated classes named D₁ and D₂. For the D₁ receptor class, only two subtypes are found in mammals, the D_1A and D_1B, receptors, whereas additional subtypes, named D_1C, D_1D, and D_1X, have been found in other vertebrate species. Here, we analyzed molecular phylogeny, gene synteny, and gene expression pattern of the D₁ receptor subtypes in a large range of vertebrate species, which leads us to propose a new view of the evolution of D₁ dopamine receptor genes. First, we show that D_1C and D_1D receptor sequences are encoded by orthologous genes. Second, the previously identified Cypriniform D_1X sequence is a teleost-specific paralog of the D_1B sequences found in all groups of jawed vertebrates. Third, zebrafish and several sauropsid species possess an additional D₁-like gene, which is likely to form another orthology group of vertebrate ancestral genes, which we propose to name D_1E. Ancestral jawed vertebrates are thus likely to have possessed four classes of D₁ receptor genes—D_1A, D_1B(X), D_1C(D), and D_1E—which arose from large-scale gene duplications. The D_1C receptor gene would have been secondarily lost in the mammalian lineage, whereas the D_1E receptor gene would have been lost independently in several lineages of modern vertebrates. The D_1A receptors are well conserved throughout jawed vertebrates, whereas sauropsid D_1C receptors have rapidly diverged, to the point that they were misidentified as D_1D. The functional significance of the D_1C receptor loss is not known. It is possible that the function may have been substituted with D_1A or D_1B receptors in mammals, following the disappearance of D_1C receptors in these species.

Characterisation of AmphiAmR4, an amphioxus (Branchiostoma floridae) α₂-adrenergic-like G-protein-coupled receptor

Little is known about the evolutionary relationship between vertebrate adrenergic receptors and invertebrate octopamine and tyramine receptors. The complexity of the adrenergic signalling system is believed to be an innovation of the vertebrate lineage but the presence of noradrenaline has been reported in some invertebrate species. The cephalochordate, amphioxus (Branchiostoma floridae), is an ideal model organism for studying the evolution of vertebrate GPCRs, given its unique position at the base of the chordate lineage. Here, we describe the pharmacological characterisation and second messenger coupling abilities of AmphiAmR4, which clusters with α₂-adrenergic receptors in a phylogenetic tree but also shares a high sequence similarity to invertebrate octopamine/tyramine receptors in both BLAST and Hidden Markov Model analyses. Thus, it was of particular interest to determine if AmphiAmR4 displayed similar functional properties to the vertebrate α₂-adrenergic receptors or to invertebrate octopamine or tyramine receptors. When stably expressed in Chinese hamster ovary (CHO) cells, noradrenaline couples the receptor to both the activation of adenylyl cyclase and to the activation of the MAPKinase pathway. Pharmacological studies with a wide range of agonists and antagonists suggest that AmphiAmR4 functions as an α₂-adrenergic-like receptor when expressed in CHO cells.

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.

The evolution of dopamine systems in chordates

Dopamine (DA) neurotransmission in the central nervous system (CNS) is found throughout chordates, and its emergence predates the divergence of chordates. Many of the molecular components of DA systems, such as biosynthetic enzymes, transporters, and receptors, are shared with those of other monoamine systems, suggesting the common origin of these systems. In the mammalian CNS, the DA neurotransmitter systems are diversified and serve for visual and olfactory perception, sensory–motor programming, motivation, memory, emotion, and endocrine regulations. Some of the functions are conserved among different vertebrate groups, while others are not, and this is reflected in the anatomical aspects of DA systems in the forebrain and midbrain. Recent findings concerning a second tyrosine hydroxylase gene (TH2) revealed new populations of DA-synthesizing cells, as evidenced in the periventricular hypothalamic zones of teleost fish. It is likely that the ancestor of vertebrates possessed TH2 DA-synthesizing cells, and the TH2 gene has been lost secondarily in placental mammals. All the vertebrates possess DA cells in the olfactory bulb, retina, and in the diencephalon. Midbrain DA cells are abundant in amniotes while absent in some groups, e.g., teleosts. Studies of protochordate DA cells suggest that the diencephalic DA cells were present before the divergence of the chordate lineage. In contrast, the midbrain cell populations have probably emerged in the vertebrate lineage following the development of the midbrain–hindbrain boundary. The functional flexibility of the DA systems, and the evolvability provided by duplication of the corresponding genes permitted a large diversification of these systems. These features were instrumental in the adaptation of brain functions to the very variable way of life of vertebrates.

Amphioxus expresses both vertebrate-type and invertebrate-type dopamine D(1) receptors

The cephalochordate amphioxus (Branchiostoma floridae) has recently been placed as the most basal of all the chordates, which makes it an ideal organism for studying the molecular basis of the evolutionary transition from invertebrates to vertebrates. The biogenic amine, dopamine regulates many aspects of motor control in both vertebrates and invertebrates, and in both cases, its receptors can be divided into two main groups (D1 and D2) based on sequence similarity, ligand affinity and effector coupling. A bioinformatic study shows that amphioxus has at least three dopamine D1-like receptor sequences. We have recently characterized one of these receptors, AmphiD1/β, which was found to have high levels of sequence similarity to both vertebrate D1 receptors and to β-adrenergic receptors, but functionally appeared to be a vertebrate-type dopamine D(1) receptor. Here, we report on the cloning of two further dopamine D(1) receptors (AmphiAmR1 and AmphiAmR2) from adult amphioxus cDNA libraries and their pharmacological characterisation subsequent to their expression in cell lines. AmphiAmR1 shows closer structural similarities to vertebrate D(1)-like receptors but shows some pharmacological similarities to invertebrate "DOP1" dopamine D(1)-like receptors. In contrast, AmphiAmR2 shows closer structural and pharmacological similarities to invertebrate "INDR"-like dopamine D(1)-like receptors.