Kainate-binding, human CNS receptors of the EAA2 family

#This invention is concerned with applications of recombinant DNA technology in the field of neurobiology. More particularly, the invention relates to the cloning and expression of DNA coding for excitatory amino acid (EAA) receptors, especially human EAA receptors.

#In the mammalian central nervous system (CNS), the transmission of nerve impulses is controlled by the interaction between a neurotransmitter substance released by the "sending" neuron and a surface receptor on the "receiving" neuron. L-glutamate is the most abundant neurotransmitter in the CNS, and mediates the major excitatory pathway in vertebrates. Glutamate is therefore referred to as an excitatory amino acid (EAA) and the receptors which respond to it are variously referred to as glutamate receptors, or more commonly as EAA receptors.

#Using tissues isolated from mammalian brain, various synthetic EAA receptor agonists, knowledge of EAA receptor pharmacology has been refined somewhat. Members of the EAA receptor family are now grouped into three main types based on differential binding to such agonists. One type of EAA receptor, which in addition to glutamate also binds the agonist NMDA (N-methyl-D-aspartate), is referred to as the NMDA type of EAA receptor. Two other glutamate-binding types of EAA receptor, which do not bind NMDA, are named according to their preference for binding with two other EAA receptor agonists, namely AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate), and kainate. Particularly, receptors which bind glutamate but not NMDA, and which bind with greater affinity to kainate than to AMPA, are referred to as kainate type EAA receptors. Similarly, those EAA receptors which bind glutamate but not NMDA, and which bind AMPA with greater affinity than kainate are referred to as AMPA type EAA receptors.

#FIG. 1 provides the nucleotide sequence (SEQ ID. NO. 1) of DNA coding for an excitatory amino acid receptor of the present invention, and the deduced amino acid sequence thereof (SEQ ID. NO. 2);

#FIG. 2 illustrates schematically a PCR-based strategy for amplifying the DNA sequence illustrated in FIG. 1;

#FIGS. 3(1) and 3(2) illustrate with linear plasmid maps the strategy used to construct expression vectors harbouring the DNA sequence illustrated in FIG. 1;

#The invention relates to excitatory amino acid (EAA) receptors of human origin, and is directed more particularly to a novel family of kainate-type human EAA receptors, herein designated the human EAA2 receptor family. As used herein, the term "human EAA2 receptor" is intended to embrace the human EAA2a receptor, and kainate-binding variants of the EAA2a receptor that are structurally related thereto, i.e. have at least 95% homology therewith, including naturally occurring and synthetically derived variants of the EAA2a receptor. Naturally occurring variants of the human EAA2a receptor include particularly the receptors herein designated human EAA2b receptor, and human EAA2c receptor. As used herein, the term "kainate-binding" refers to receptor variants and receptor fragments that display greater binding affinity for kainate than for either glutamate, AMPA or NMDA, as determined in assays of conventional design, such as the assays herein described.

#The particular human EAA receptor designated EAA2a is a protein characterized structurally as a single polypeptide chain that is produced initially in precursor form bearing an 18 residue N-terminal signal peptide, and is transported to the cell surface in mature form, lacking the signal peptide and consisting of 962 amino acids arranged in the sequence illustrated by single letter code in FIG. 1. Unless otherwise stated, amino acid residues of the EAA2a receptor are numbered with reference to the mature protein sequence. With respect to structural domains of the receptor, hydropathy analysis reveals four putative transmembrane domains, one spanning residues 528-547, inclusive, (TM-1), another spanning residues 572-590 (TM-2), a third spanning residues 601-619 (TM-3) and the fourth spanning residues 786-806 (TM-4). Based on this assignment, it is likely that the human EAA2 receptor structure, in its natural membrane-bound form, consists of a 527 amino acid N-terminal extracellular domain, followed by a hydrophobic region containing four transmembrane domains and an extracellular, 156 amino acid C-terminal domain.

#As shown in FIG. 4, structurally related variants of the EAA2a receptor, which occur naturally in human brain tissue, have also been identified. As deduced from nucleotide sequences of the genes coding for them, these variants differ structurally therefrom by the insertion of one additional amino acid between positions 473 and 474 of EAA2a, in the case of EAA2b. Another variant, designated EAA2c, differs from EAA2a by fifteen amino acids in the N-terminal region (FIG. 4). A further variant, designated EAA2d, differs from EAA2a also in the N-terminal region, and has a seven amino acid deletion (FIG. 4).