Molecular Identification of the Human GABA
B
R2:
Cell Surface Expression and Coupling to Adenylyl
Cyclase in the Absence of GABA
B
R1
Stella C. Martin, Shelley J. Russek, and David H. Farb
1
Laboratory of Molecular Neurobiology, Department of Pharmacology, Boston University School
of Medicine, 715 Albany Street, Boston, Massachusetts 02118-2394
We have identified a gene encoding a GABA
B
receptor, the
human GABA
B
R2, located on chromosome 9q22.1, that is
distinct from the recently reported rat GABA
B
R1. GABA
B
R2
structurally resembles GABA
B
R1 (35% identity), having
seven transmembrane domains and a large extracellular
region, but differs in having a longer carboxy-terminal tail.
GABA
B
R2 is localized to the cell surface in transfected
COS cells, and negatively couples to adenylyl cyclase in
response to GABA, baclofen, and 3-aminopropyl(meth-
yl)phosphinic acid in CHO cells lacking GABA
B
R1. Ba-
clofen action is inhibited by the GABA
B
R antagonist,
2-hydroxysaclofen. The human GABA
B
R2 and GABA
B
R1
genes are differentially expressed in the nervous system,
with the greatest difference being detected in the striatum
in which GABA
B
R1 but not GABA
B
R2 mRNA transcripts
are detected. GABA
B
R2 and GABA
B
R1 mRNAs are also
coexpressed in various brain regions such as the Purkinje
cell layer of the cerebellum. Identification of a functional
homomeric GABA
B
R2 coupled to adenylyl cyclase sug-
gests that the complexity of GABA
B
pharmacological data
is at least in part due to the presence of more than one
receptor and opens avenues for future research leading to
an understanding of metabotropic GABA receptor signal
transduction mechanisms.
INTRODUCTION
The type Aand B ␥-aminobutyric acid (GABA) recep-
tors mediate most synaptic inhibition in the central
nervous system (Rabow et al., 1995; Bowery, 1989; 1993).
Whereas GABA action at ionotropic type A receptors
initiates fast inhibition through an increase in the chlo-
ride ion conductance, metabotropic type B receptors
mediate slow inhibition by interacting with guanine-
nucleotide-binding (G) proteins to inhibit adenylyl cy-
clase activity, activate potassium channels, and inacti-
vate voltage-dependent calcium channels. Awide range
of actions have been attributed to GABA
B
receptor
stimulation such as: analgesia, catatonia, modulation of
gastric motility, reduction of memory consolidation and
retention, and suppression of panic attacks (Bowery,
1989).
Kaupmann et al. (1997) described the expression
cloning of GABA
B
R1 with rat cortex and cerebellum
cDNA libraries, using detection by binding of the
radiolabeled GABA
B
antagonist, CGP64213. GABA
B
R1
is a seven transmembrane domain receptor with homol-
ogyto the metabotropicglutamatereceptorsand couples
to adenylyl cyclase in transfected HEK293 cells. Two
variants differing only in their N-terminal extracellular
region were found, GABA
B
R1a and b. Pharmacological
studies have suggested a heterogeneity in the GABA
B
R,
which could be due to more than one receptor (Bowery,
1993; Cunningham and Enna, 1996). Further support for
this notion comes from the incomplete correlation be-
tween the binding affinities of native GABA
B
R and
recombinant GABA
B
R1 (Kaupmann et al., 1997). A fur-
ther level of functional complexity has been revealed by
the demonstration that GABA
B
R1 does not localize to
the plasma membrane in some transfected cells (Couve
et al., 1998). This suggests that GABA
B
R1 requires
additional information, perhaps in the form of a traffick-
ing factor or binding partner, to reach the cell surface.
A bioinfomatics-based approach was used to search
for homologs of the rat GABA
B
R1. In addition to
identifying the human GABA
B
R1, we have discovered
an additional gene that encodes the human GABA
B
R2, a
seven transmembrane receptor that negatively couples
1
To whom correspondence and reprint requests should be ad-
dressed. Fax: (617) 638-4329. E-mail: dfarb@bu.edu.
MCN
Molecular and Cellular Neuroscience
13, 180–191 (1999)
Article ID
mcne.1999.0741,
available online at http://www.idealibrary.com on
180
1044-7431/99 $30.00
Copyright
1999 by Academic Press
All rights of reproduction in any form reserved.