NOVEL GABA-B RECEPTOR

Abstract: The invention provides isolated polynucleotide molecules encoding a novel neuropeptide GABA-B receptor (designated GABA-B). These isolated polynucleotide molecules can be used to express the receptor in cells which can then be used to screen compounds for agonist and antagonist activity.

Description: NOVEL GABA-B RECEPTOR Field of the Invention: The present invention relates to isolated polynucleotide molecules which encode a novel transmembrane G-protein coupled receptor designated GABA-B2. The novel receptor appears to be activated by the neurotransmitter γ-amino butyric acid (GABA). Background of the Invention: γ-amino butyric acid (GABA) is the principal inhibitory neurotransmitter in the brain, whose action is mediated by two types of receptors, GABA-A and GABA-B. GABAergic inhibitory neurons typically form short pathways (e.g. from striatum to substantia nigra and from cerebellar cortex to deep cerebellar nuclei), although at least one long pathway projecting from the posterior hypothalamus to the cerebral cortex has also been recognised. This long pathway is believed to provide a direct pathway by which limbic, emotional and visceral information may be transferred to the cortex (Vincent et al, Science 220: 1309-1311, 1993). GABA-B receptors, such as the GABA-Bla and GABA-Blb receptors, are predominantly present in the brain where they are believed to play a major role in learning and memory. In view of these functions and the known benefit of GABA-B agonists (e.g. baclofen) in the treatment of spasticity, anxiety and depression, there is considerable interest in isolating genes encoding GABA-B receptor subtypes so as to enable the recombinant production of GABA-B receptors for the development of novel therapeutics. Summary of the Invention: In a first aspect, the present invention provides an isolated polynucleotide molecule encoding a GABA-B2 receptor or a functionally equivalent fragment thereof. Preferably, the encoded GABA-B2 receptor is characterised by the N- terminal amino acid sequence: MASPRSSGQP (SEQ ID NO: 1) More preferably, the isolated polynucleotide molecule encodes a human GABA-B2 receptor of about 941 amino acids. Most preferably, the isolated polynucleotide molecule encodes a GABA-B2 receptor having an amino acid sequence substantially corresponding to that shown as SEQ ID NO: 2. The polynucleotide molecule of the first aspect may comprise a nucleotide sequence substantially corresponding to, or showing at least 90% (more preferably, at least 95%) homology to that shown at nucleotides 1 to 3256 or nucleotides 140 to 2962 of SEQ ID NO: 3 or any portion thereof encoding a functionally equivalent GABA-B2 receptor fragment. The isolated polynucleotide molecule may be incorporated into plasmids or expression vectors (including viral vectors), which may then be introduced into suitable bacterial, yeast, insect and mammalian host cells. Such host cells may be used to express the GABA-B2 receptor encoded by the isolated polynucleotide molecule. Accordingly, in a second aspect, the present invention provides a mammalian, insect, yeast or bacterial host cell transformed with the polynucleotide molecule of the first aspect. In a third aspect, the present invention provides a method of producing GABA-B2 receptors or functionally equivalent fragments thereof, comprising culturing the host cell of the second aspect under conditions enabling the expression of GABA-B2 receptors or functionally equivalent fragments thereof. Preferably, the host cell is mammalian or of insect origin. Where the cell is mammalian, it is presently preferred that it be a Chinese hamster ovary (CHO) cell, monkey kidney (COS) cell or human embryonic kidney 293 cell. Where the cell is of insect origin, it is presently preferred that it be an insect Sf9 cell. In a preferred embodiment, the GABA-B2 receptors or fragments thereof are expressed onto the surface of the host cell. By using the polynucleotide molecule of the present invention it is possible to obtain GABA-B2 receptor protein or fragments thereof in a substantially pure form. Accordingly, in a fourth aspect, the present invention provides a GABA-B2 receptor or a functionally equivalent fragment of said receptor, in a substantially pure form. In a fifth aspect, the present invention provides an antibody capable of specifically binding to the GABA-B2 receptor of the fourth aspect. Such antibodies may be produced by any of the methods routine to the art. In a sixth aspect, the present invention provides a non-human animal transformed with a polynucleotide molecule according to the first aspect of the present invention. In a seventh aspect, the present invention provides a method for detecting agonist or antagonist agents of a GABA-B2 receptor, comprising contacting a GABA-B2 receptor, functionally equivalent fragment thereof or a cell transfected with and expressing the polynucleotide molecule of the first aspect, with a test agent under conditions enabling the activation of a GABA- B2 receptor, and detecting an increase or decrease in activity of the GABA-B2 receptor or functionally equivalent fragment thereof. An increase or decrease in activity of the receptor or functionally equivalent fragment thereof may be detected by measuring changes in cAMP production, Ca 2+ levels or IP3 turnover after activating the receptor molecule with specific agonists or antagonists. In a further aspect, the present invention provides an oligonucleotide or polynucleotide probe comprising a nucleotide sequence of 10 or more nucleotides, the probe comprising a nucleotide sequence such that the probe specifically hybridises to the polynucleotide molecule of the first aspect under high stringency conditions (Sambrook et al., Molecular cloning: a laboratory manual, Second Edition, Cold Spring Harbor Laboratory Press). In a still further aspect, the present invention provides an antisense oligonucleotide or polynucleotide molecule comprising a nucleotide sequence capable of specifically hybridising to an mRNA molecule which encodes a GABA-B2 receptor so as to prevent translation of the mRNA molecule. Such antisense oligonucleotide or polynucleotide molecules may include a ribozyme region to catalytically inactivate mRNA to which it is hybridised. The polynucleotide molecule of the first aspect of the invention may be a dominant negative mutant which encodes a gene product causing an altered phenotype by, for example, reducing or eliminating the activity of endogenous GABA-B2 receptors. The term "substantially corresponding" as used herein in relation to amino acid sequences is intended to encompass minor variations in the amino acid sequences which do not result in a decrease in biological activity of the GABA-B2 receptor. These variations may include conservative amino acid substitutions. The substitutions envisaged are:- G, A, V, I, L, M; D, E; N, Q; S, T; K, R, H; F, Y, W, H; and P, Nα-alkalamino acids. The term "substantially corresponding" as used herein in relation to nucleotide sequences is intended to encompass minor variations in the nucleotide sequences which due to degeneracy in the DNA code do not result in a change in the encoded protein. Further, this term is intended to encompass other minor variations in the sequence which may be required to enhance expression in a particular system but in which the variations do not result in a decrease in biological activity of the encoded protein, The term "functionally equivalent fragment/s" as used herein is intended to refer to fragments of the GABA-B2 receptor that exhibit binding specificity and activity that is substantially equivalent to the GABA-B2 receptor from which it/they is/are derived. Throughout this specification, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. Reference to percent homology made in this specification have been calculated using the BLAST program blastn as described by Altschul, S.F. et al., "Capped BLAST and PSI-BLAST: a new generation of protein database search programs", Nucleic Acids Research, Vol. 25, No. 17, pp. 3389-3402 (1997). Brief description of the accompanying Figures: Figure 1 provides the nucleotide sequence of a cDNA encoding the human GABA-B2 receptor and includes the predicted amino acid sequence. Figure 2 shows the degree of identity between the predicted amino acid sequence of the human GABA-B2 and GABA-Blb receptors, Detailed Disclosure of the Invention: Human GABA-B2 receptor cDNA A human hippocampus cDNA library (Stratagene) was screened under low stringency conditions with a 184 bp 32 P-labelled fragment (corresponding to nucleotides 2051 to 2235 of SEQ ID NO: 3) originated from a human brain EST clone (z43654). A cDNA clone encoding a complete human GABA-B gene was obtained from the screen. The nucleotide sequence of the cDNA clone is shown as SEQ ID NO: 3 and encodes a protein of 941 amino acids (SEQ ID NO: 2) consisting of a large extracellular domain (amino acids 1 to 480) followed by 7 hydrophobic regions (amino acids 481 to 494, 518 to 545, 558 to 578, 597 to 618, 653 to 676, 691 to 713 and 719 to 743) typical of G- protein coupled receptors. Sequence comparison with other G-protein coupled receptors identify GABA-B la, GABA-B lb (Figure 1) and metabotropic glutamate receptors (Kaupman, K. et al., Nature 386: 239-246, 20 March 1997 and Pin, J. et al, Neuropliarmacology 34: 1-26, 1995) as the most closely related group. Northern analysis has identified brain as the tissue with the highest expression of the human GABA-B2 mRNA. In particular, Northern analysis experiments using multiple tissue Northern blots (Clontech) identified high levels of expression of the human GABA-B2 mRNA in the cerebellum, cerebral cortex, occipital pole, frontal lobe and temporal lobe. Lower levels of expression were seen in the thalamus, amygldala, hippocampus, substantia nigra, putamen, subthalamic nucleus, caudate nucleus, and medulla. No apparent expression was seen in the spinal cord and corpus callosum. Further, in situ hybridisation of rat brain sections has identified discrete areas of expression in the hippocampus, amygdala, the piriform cortex and also the hypothalamus. This mRNA distribution is consistent with the expression of other subtypes of the GABA-B receptor family. Chromosomal Localisation of Human GABA-B2 receptor gene In order to determine the chromosomal localisation of the human GABA-B2 receptor gene, the complete cDNA clone was nick-translated with biotin-14-dAPT and hybridised in situ at a final concentration of 5 ng/ml to metaphase chromosomal spreads from two normal males. The fluoerescence in situ hybridisation (FISH) method was modified from that previously described (Callen, DF et al., Ann Genet 33: 219-221 1990) in that chromosomes were stained before analysis with both propidium iodide (as counterstain) and DAPI (for chromosome identification). Images of metaphase preparations were captured by a CCD camera and computer enhancement software. Twenty metaphases from a first normal male were examined for fluorescent signal. All of these metaphases showed signal on one or both chromatids of chromosome 9 in the region 9q21. There was a total of 7 nonspecific background dots observed in these 20 metaphases. A similar result was obtained from hybridisation of the probe to 20 metaphases from a second normal male. It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims: Claims: 1. An isolated polynucleotide molecule encoding a GABA-B receptor or a functionally equivalent fragment thereof, wherein the encoded GABA-B receptor is characterised by the N-terminal amino acid sequence: MASPRSSGQP (SEQ ID NO: 1). 2. A polynucleotide molecule according to claim 1, wherein the polynucleotide molecule encodes a GABA-B receptor of human origin of about 941 amino acids in length. 3. A polynucleotide molecule according to claim 2, wherein the polynucleotide molecule encodes a human GABA-B receptor having an amino acid sequence substantially corresponding to that shown as SEQ ID NO: 2. 4. A polynucleotide molecule encoding a GABA-B receptor, wherein the polynucleotide molecule comprises a nucleotide sequence showing at least 90% homology to that shown at nucleotides 1 to 3256 or nucleotides 140 to 2962 of SEQ ID NO: 3 or any portion thereof encoding a functionally equivalent GABA-B receptor fragment. 5. A polynucleotide molecule according to claim 4, wherein the polynucleotide molecule comprises a nucleotide sequence showing at least 95% homology to that shown at nucleotides 1 to 3256 or nucleotides 140 to 2962 of SEQ ID NO: 3 or any portion thereof encoding a functionally equivalent GABA-B receptor fragment. 6. A polynucleotide molecule according to claim 4, wherein the polynucleotide molecule comprises a nucleotide sequence substantially corresponding to that shown at nucleotides 1 to 3256 or nucleotides 140 to 2962 of SEQ ID NO: 3 or any portion thereof encoding a functionally equivalent GABA-B receptor fragment. 7. A plasmid or expression vector including a polynucleotide molecule according to any one of claims 1 to 6. 8. A host cell transformed with a polynucleotide molecule according to any one of claims 1 to 6 or a plasmid or expression vector according to claim 7. 9. A host cell according to claim 8, wherein the cell is a mammalian or insect cell. 10. A host cell according to claim 9, wherein the cell is a Chinese hamster ovary (CHO) cell, human embryonic kidney (HEK) 293 cell or an insect Sf9 cell. 11. A host cell according to any one of claims 8 to 10, wherein the cell expresses the GABA-B receptor or functionally equivalent fragment thereof onto the cell's surface. 12. A GABA-B receptor which is characterised by the N-terminal amino acid sequence: MASPRSSGQP (SEQ ID NO:l), in a substantially pure form. 13. A receptor according to claim 12, wherein said receptor is a human receptor of about 941 amino acids. 14. A receptor according to claim 13, wherein said receptor has an amino acid sequence substantially corresponding to that shown as SEQ ID NO: 2. 15. An antibody or fragment thereof which specifically binds to a GABA-B receptor according to any one of claims 12 to 14. 10. A non-human animal transformed with a polynucleotide molecule according to any one of claims 1 to 6 or a plasmid or expression vector according to claim 7. 17. A method for detecting agonist or antagonist agents of a GABA-B receptor, comprising contacting a GABA-B receptor according to any one of claims 12 to 14 or a host cell according to any one of claims 8 to 11, with a test agent under conditions enabling the activation of said receptor, and detecting an increase or decrease in the receptor activity. 18. An oligonucleotide or polynucleotide probe comprising a nucleotide sequence of 10 or more nucleotides, the probe comprising a nucleotide sequence such that the probe specifically hybridises to the polynucleotide molecule according to any one of claims 1 to 6 under high stringency conditions. 19. An antisense oligonucleotide or polynucleotide molecule comprising a nucleotide sequence capable of specifically hybridising to an mRNA molecule which encodes a GABA-B receptor encoded by the polynucleotide molecule according to any one of claims 1 to 6, so as to prevent translation of the mRNA molecule. 20. A method of producing GABA-B receptors or functionally equivalent fragments thereof according to any one of claims 12 to 14, comprising culturing a host cell according to any one of claims 8 to 11 under conditions enabling the expression of GABA-B receptors or functionally equivalent fragments thereof, and optionally recovering the receptors or functionally equivalent fragments thereof.

Inventors: HERZOG HERBERT, HERZOG, HERBERT
Applicants: GARVAN INST MED RES, HERZOG HERBERT, GARVAN INSTITUTE OF MEDICAL RESEARCH, HERZOG, HERBERT
Application year: 1999
Main classification: C12N15
IPC codes: A01K67, C07K14, C07K16, C12N5, C12N15, C12P21, C12Q1, G01N33