Cloning of a Molluscan G Protein alpha Subunit of the Gq Class which is Expressed Differentially in Identified Neurons

Expression of G Proteins in the Eyes and Parietovisceral Ganglion of the Bay Scallop Argopecten irradians

A multitude of image-forming eyes are spread across the bodies of certain invertebrates. Recent efforts have characterized how these eyes function, but less progress has been made toward describing the neural structures associated with them. Scallops, for example, have a distributed visual system that includes dozens of eyes whose optic nerves project to the lateral lobes of the parietovisceral ganglion (PVG). To identify sensory receptors and chemical synapses associated with the scallop visual system, we studied the expression of four G protein α subunits (Gα_i, Gα_o, Gα_q, and Gα_s) in the eyes and PVG of the bay scallop Argopecten irradians (Lamarck, 1819). In the eyes of A. irradians, we noted expression of Gα_o by the ciliary photoreceptors of the distal retina, expression of Gα_q by the rhabdomeric photoreceptors of the proximal retina, and the expression of Gα_o and Gα_q by the cells of the cornea; we did not, however, detect expression of Gα_i or Gα_s in the eyes. In the PVG of A. irradians, we noted widespread expression of Gα_i, Gα_o, and Gα_q. The expression of Gα_s was limited to fine neurites in the lateral and ventral central lobes, as well as large unipolar neurons in the dorsal central lobes. Our findings suggest that light detection by the eyes of A. irradians is conferred primarily by photoreceptors that express Gα_o or Gα_q, that the corneal cells of scallops may contain sensory receptors and/or receive neural input, and that G protein labeling is useful for visualizing substructures and identifying specific populations of cells within the nervous systems of invertebrates.

Immunolocalization of a Galphaq protein to the chemosensory organs of Dipolydora quadrilobata (polychaeta: spionidae)

Chemoreception in marine invertebrates mediates a variety of ecologically important behaviors including defense, reproduction, larval settlement and recruitment, and feeding. The sensory pathways that regulate deposit-feeding activity by polychaetes living in sedimentary habitats are of particular interest because such feeding has profound effects on the physical and chemical properties of the habitat. Nevertheless, little is known concerning the molecular mechanisms of chemical signal transduction associated with deposit feeding and other behaviors in polychaetes. Chemosensory-based feeding behaviors are typically regulated by G-protein-coupled signal transduction pathways. However, the presence and role of such pathways have not been demonstrated in marine polychaetes. Methodologies involving degenerate primer-based reverse transcription with the polymerase chain reaction and rapid amplification of cDNA ends were used to identify and characterize a Galphaq subunit expressed in the feeding palps of the spionid polychaete Dipolydora quadrilobata. The D. quadrilobata Galphaq protein had high sequence similarity with previously reported Galphaq subunits from both invertebrate and vertebrate taxa. Immunhistochemistry and immunocytochemistry were used with confocal laser scanning microscopy and transmission electron microscopy to visualize the distribution of a Galphaq antibody in whole worms and in cilia of the feeding palps. Galphaq immunoreactivity was concentrated in the nuchal organs, food-groove cilia, and lateral/abfrontal cilia of the feeding palps. Because these structures are known to be involved in chemoreception, we propose that Galphaq isolated from D. quadrilobata is a key component of chemosensory signal transduction pathways in this species.

Gene identification and evidence for expression of G protein alpha subunits, phospholipase C, and an inositol 1,4,5-trisphosphate receptor in Aplysia californica rhinophore

In the marine mollusk Aplysia californica, waterborne protein pheromones that are released during egg laying act in concert to stimulate mate attraction. However, molecular information concerning the cellular receptors and signaling mechanisms that may be involved in waterborne peptide and protein pheromonal communication is lacking. As a first step toward examining whether members of the G protein family and phosphoinositide signaling pathway are present in the primary peripheral chemosensory organs (i.e., rhinophores), we isolated five full-length cDNA clones from an A. californica central nervous system cDNA library. These clones encoded (1) the G protein alpha subunits of the Gq, Gi, and Go families, (2) a protein with homology to phospholipase C (PLC) isoforms, and (3) an inositol 1,4,5-trisphosphate receptor (IP3R). The expression of these genes was examined using laser capture microdissection/reverse transcription-polymerase chain reaction and in situ hybridization. All of them are expressed in the rhinophore sensory epithelium, suggesting that Galphaq, Galphai, Galphao, PLC-like protein, and IP3R may be involved in waterborne protein pheromone detection in Aplysia-possibly via a phosphoinositide signaling mechanism.

Diversity of G proteins in Lepidopteran cell lines: partial sequences of six G protein alpha subunits

The aim of this work was to sample the diversity of G protein alpha subunits in lepidopteran insect cell lines. Here we report the amplification by degenerate PCR of partial sequences representing six G protein alpha subunits from three different lepidopteran insect cell lines. Sequence comparisons with known G protein alpha subunits indicate that the Sf9, Ld and High Five cell lines each contain (at least) one Galpha(q)-like and one Galpha(i)-like Galpha subunit. All six PCR products are unique at the nucleotide level, but the translation products of the three Galpha q-like partial clones (Sf9-Galpha 1, Ld-Galpha 1, and Hi5-Galpha 1) are identical, as are the translation products of the three Galpha i-like partial clones (Sf9-Galpha 2, Ld-Galpha 2, and Hi5-Galpha 2). Both the Galpha(q)-like and Galpha(i)-like translation products are identical to known Galpha subunits from other Lepidoptera, are highly similar (88-98%) to Galpha subunits from other invertebrates including mosquitoes, fruit flies, lobsters, crabs, and snails, and are also highly similar (88-90%) to known mammalian Galpha subunits. Identification of G protein alpha subunits in lepidopteran cell lines will assist in host cell line selection when insect cell lines are used for the pharmacological analysis of human GPCRs.

Expression pattern in the antennae of a newly isolated lepidopteran Gq protein alpha subunit cDNA

From the antennae of the moth Mamestra brassicae, we have identified a lepidopteran G protein alpha subunit belonging to the Gq family, through immunological detection in crude antennal extract and antennal primary cell cultures, followed by molecular cloning. The complete cDNA sequence (1540 bp) contains an open reading frame encoding a protein of 353 amino acids. This deduced sequence possesses all of the characteristics of the Gq family and shares a very high degree of amino-acid sequence identity with vertebrate (80% with mouse or human Gqalpha) and invertebrate subunits (varying between 60 and 87% for Gqalpha from organisms as diverse as sponge and Drosophila). The expression pattern of the Gq subunit in adult antennae was associated with the olfactory sensilla suggesting a specific role in olfaction. These data provide molecular evidence for a component of the phosphoinositide signaling pathway in moth antennae: this G protein alpha subunit may be involved in the olfaction transduction process through interaction with G-protein-coupled receptors, stimulating the phospholipase C mediated second messenger pathway.

Molecular cloning and characterization of an invertebrate homologue of a neuropeptide Y receptor

Neuropeptide Y is an abundant and physiologically important peptide in vertebrates having effects on food intake, sexual behaviour, blood pressure and circadian rhythms. Neuropeptide Y homologues have been found in invertebrates, where they are very likely to play similar, important roles. Although five neuropeptide Y-receptor subtypes have been identified in mammals, none has been reported from invertebrates. Here we describe the cloning of a neuropeptide Y-receptor from the brain of the snail Lymnaea stagnalis. The identity of the receptor was deduced by expressing the neuropeptide Y-receptor-encoding cDNA in Chinese Hamster Ovary cells, which were subsequently challenged with size-fractionated Lymnaea brain extracts. An active peptide, selected on the basis of its ability to induce changes in cAMP levels, was purified to homogeneity, analysed by mass spectrometry and amino acid sequence determination, and turned out to be a Lymnaea homologue of neuropeptide Y.

Evolutionary analysis of G-proteins in early metazoans: cloning of alpha- and beta-subunits from the sponge Geodia cydonium

G-protein-coupled (seven-transmembrane segment)-receptors represent a major group of metazoan receptors, involved in transduction of extracellular signals. The G-proteins, which are made up of Galpha/beta/gamma-subunits, link the receptors to the effector system(s). To analyze the phylogenetic relationships among the metazoan alpha-subunits of G-proteins, cDNAs of alpha-subunits were isolated from Geodia cydonium, a marine sponge belonging to the lowest metazoan phylum, Porifera. One encodes a putative isotype of a stimulator of the adenylyl cyclase (Galpha s), another one a putative inhibitor of the adenylyl cyclase (Galpha i/o) and the third one a putative activator of phospholipase C (Galpha q). In addition one putative beta-subunit was cloned from the same species. The deduced amino acid sequences of the sponge Galpha s -(putative Mr 44749), the Galpha i/o -(Mr 41064) and the Galpha q subunits (Mr 41363) were found to display high similarity with the corresponding sequences from higher Metazoa, and are only distantly related to those of slime mold, yeast or plants. Of lower similarity are the sequences of the beta-subunits among animals and plants, thus not allowing robust grouping. These data demonstrate that the phylogenetic relationships, obtained from analyses of the alpha subunits from metazoan G-proteins, support the conclusion that all metazoan phyla, including the Porifera are of monophyletic origin.

Functional characterisation of a 5-HT2 receptor cDNA cloned from Lymnaea stagnalis

A G-protein-coupled receptor (5-HT2Lym) resembling members of the 5-HT2 receptor subfamily was cloned from the mollusc Lymnaea stagnalis. Serotonin induces a concentration-dependent increase in intracellular inositol phospates in HEK293 cells expressing this receptor (EC50 = 114 nM). 5-HT2Lym differs from mammalian 5-HT2 receptors by the presence of a large amino-terminal region. This large domain appears to preclude an adequate level of expression of 5-HT2Lym in HEK293. Therefore, we constructed a cDNA encoding an amino-terminally truncated receptor (delta N-5-HT2Lym) that appeared to be much better expressed in HEK293 cells. delta N-5-HT2Lym-expressing cells exhibit a serotonin-induced stimulation of phosphatidylinositol bisphosphate hydrolysis (EC50 = 11.4 nM) and a high-affinity binding of the 5-HT2-selective antagonist [3H]mesulergine (Kd = 4 nM). Inhibition of this binding by several 5-HT2 antagonists and agonists revealed a pharmacological profile most closely resembling those of 5HT2Dro, 5-HT2B and 5-HT2C.

Voltage gated calcium channels in molluscs: classification, Ca2+ dependent inactivation, modulation and functional roles

Molluscan neurons and muscle cells express transient (T-type like) and sustained LVA calcium channels, as well as transient and sustained HVA channels. In addition weakly voltage sensitive calcium channels are observed. In a number of cases toxin or dihydropyridine sensitivity justifies classification of the HVA currents in L, N or P-type categories. In many cases, however, pharmacological characterization is still preliminary. Characterization of novel toxins from molluscivorous Conus snails may facilitate classification of molluscan calcium channels. Molluscan preparations have been very useful to study calcium dependent inactivation of calcium channels. Proposed mechanisms explain calcium dependent inactivation through direct interaction of Ca2+ with the channel, through dephosphorylation by calcium dependent phosphatases or through calcium dependent disruption of connections with the cytoskeleton. Transmitter modulation operating through various second messenger mediated pathways is well documented. In general, phosphorylation through PKA, cGMP dependent PK or PKC facilitates the calcium channels, while putative direct G-protein action inhibits the channels. Ca2+ and cGMP may inhibit the channels through activation of phosphodiesterases or phosphatases. Detailed evidence has been provided on the role of sustained LVA channels in pacemaking and the generation of firing patterns, and on the role of HVA channels in the dynamic changes in action potentials during spiking, the regulation of the release of transmitters and hormones, and the regulation of growth cone behavior and neurite outgrowth. The accessibility of molluscan preparations (e.g. the squid giant synapse for excitation release studies, Helisoma B5 neuron for neurite and synapse formation) and the large body of knowledge on electrophysiological properties and functional connections of identified molluscan neurons (e.g. sensory neurons, R15, egg laying hormone producing cells, etc.) creates valuable opportunities to increase the insight into the functional roles of calcium channels.

Heterotrimeric G proteins

Over the past year, the thrust of work in the field of heterotrimeric G proteins has been primarily in the following areas: first, resolution of their three-dimensional structures by X-ray crystallography; second, elucidation of the effect of lipid modifications on the Galpha and Ggamma subunits; third, understanding the role of the Gbetagamma dimer in stimulation of a variety of effectors following receptor activation; and fourth, identification of the points of contact among the Galpha, Gbeta, and Ggamma subunits, and between these subunits and their cognate receptor or effector molecules.

Evolution and overview of classical transmitter molecules and their receptors

All the classical transmitter ligand molecules evolved at least 1000 million years ago. With the possible exception of the Porifera and coelenterates (Cnidaria), they occur in all the remaining phyla. All transmitters have evolved the ability to activate a range of ion channels, resulting in excitation, inhibition and biphasic or multiphasic responses. All transmitters can be synthesised in all three basic types of neurones, i.e. sensory, interneurone and motoneurone. However their relative importance as sensory, interneurone or motor transmitters varies widely between the phyla. It is likely that all neurones contain more than one type of releasable molecule, often a combination of a classical transmitter and a neuroactive peptide. Second messengers, i.e. G proteins and phospholipase C systems, appeared early in evolution and occur in all phyla that have been investigated. Although the evidence is incomplete, it is likely that all the classical transmitter receptor subtypes identified in mammals, also occur throughout the phyla. The invertebrate receptors so far cloned show some interesting homologies both between those from different invertebrate phyla and with mammalian receptors. This indicates that many of the basic receptor subtypes, including benzodiazepine subunits, evolved at an early period, probably at least 800 million years ago. Overall, the evidence stresses the similarity between the major phyla rather than their differences, supporting a common origin from primitive helminth stock.

A novel G protein alpha subunit containing atypical guanine nucleotide-binding domains is differentially expressed in a molluscan nervous system

We described the characterization of a novel G protein alpha subunit, G alpha a. cDNA encoding this subunit was cloned from the central nervous system of the mollusc Lymnaea stagnalis. The deduced protein contains all characteristic guanine nucleotide-binding domains of G alpha subunits but shares only a limited degree of overall sequence identity with known subtypes (approximately 30%). Moreover, two of the nucleotide-binding domains exhibit salient deviations from corresponding sequences in other G protein alpha subunits. The A domain, determining kinetic features of the GTPase cycle, contains a markedly unique amino acid sequence (ILIIGGPGAGK). In addition, the C domain is also clearly distinct (DVAGQRSL). The presence of a leucine in this motif, instead of glutamic acid, has important implications for hypotheses concerning the GTPase mechanism. In contrast to other G alpha subtypes, G alpha a has no appropriate N-terminal residues that could be acylated. It does contain the strictly conserved arginine residue that serves as a cholera toxin substrate in G alpha s and G alpha t but lacks a site for ADP-ribosylation by pertussis toxin. In situ hybridization experiments indicate that G alpha a-encoding mRNA is expressed in a limited subpopulation of neurons within the Lymnaea brain. These data suggest that G alpha a defines a separate class of G proteins with cell type-specific functions.