Glutamate Receptors - Structures and Functions
L-Glutamate is the major excitatory neurotransmitter in the mammalian CNS, acting through both ligand gated ion channels (ionotropic receptors) and G-protein coupled (metabotropic) receptors. Activation of these receptors is responsible for basal excitatory synaptic transmission and many forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD), which are thought to underlie learning and memory. They are thus also potential targets for therapies for CNS disorders such as epilepsy and Alzheimer's diseases. A page detailing the structure and functions of various AMPA and NMDA receptor interacting proteins may also be found.
Ionotropic Glutamate Receptors
Functions of the ionotropic glutamate receptors
Glutamate is the major excitatory neurotransmitter in central nervous system (CNS) and as such the glutamate receptors play a vital role in the mediation of excitatory synaptic transmission (see animation). This process is the means by which cells in the brain (neurons) communicate with each other. An electrical impulse in one cell causes an influx of calcium ions and the subsequent release of a chemical neurotransmitter (e.g. glutamate). The transmitter diffuses across a small gap between two cells (the synaptic cleft) and stimulates (or inhibits) the next cell in the chain by interacting with receptor proteins. The specialised structure that performs this vital function is the synapse and it is in the synapse that the ionotropic glutamate receptors are generally found.
The ionotropic receptors themselves are ligand gated ion channels, ie on binding glutamate that has been released from a companion cell, charged ions such as Na+ and Ca2+ pass through a channel in the centre of the receptor complex. This flow of ions results in a depolarisation of the plasma membrane and the generation of an electrical current that is propagated down the processes (dendrites and axons) of the neuron to the next in line.
Structure of the ionotropic glutamate receptors
The ionotropic glutamate receptors are multimeric
assemblies of four or five subunits, and are subdivided into three groups
(AMPA, NMDA and Kainate receptors)
based on their pharmacology structural similarities (Figure 1). All
ionotropic glutamate receptor subunits share a common basic structure.
Like other ligand gated ion channels, such as the GABAA receptor,
the ionotropic glutamate receptor subunits possess four hydrophobic regions
within the central portion of the sequence (TMI - IV; Figure 2). However,
in contrast to other receptor subunits, the TMII domain forms a re-entrant
loop giving these receptor subunits an extracellular N-terminus and intracellular
C-terminus (Figure 2). 
In
addition, the long loop between TMIII and TMIV, which is intracellular
in other ligand gated ion channel subuhnits, is exposed to the cell surface,
and forms part of the binding domain with the C-terminal half of the N-terminus.
These two sequences show structural similarities to the bacterial periplasmic
amino-acid binding protein.
Diversity of the ionotropic glutamate receptors
The ionotropic glutamate receptors are an extremely diverse group of receptors. This diversity is generated both before and after gene transcription. Individual receptors are multimeric assemblies of subunits, transcribed from separate genes (eg NR2A-D are produced from four genes). However, following gene transcription, the resultant pre-mRNA may be modified. For instance, different regions of this mRNA molecule can be spliced together, giving rise to multiple mRNAs that are translated into different proteins. This is known as 'splice variation' and is very common among neuroreceptors. The C-terminus of the ionotropic glutamate receptors is the site of extensive splice variation (for example, see the NMDA receptors). This could have important functional consequences as the C-terminus is also the site for multiple protein-protein interations (eg GluR2/NSF). Thus different splice variants may interact differently with the same set of proteins leading to, for instance, differential subunit localisation.
A further modification leading to diversification
is RNA editing, in which selected nucleotides in the mRNA sequence transcribed
from the gene sequence are enzymatically modified, changing the amino-acid
coded for. This has fundamental consquences for the calcium permeability
of subunits such as the AMPA type subunit GluR2 and
the kainate receptor subunit GluR5. 
NMDA
receptors are composed of assemblies of NR1 subunits (Figure 3) and NR2 subunits,
which can be one of four separate gene products (NR2A-D). Expression of both subunits
are required to form functional channels. The glutamate binding domain is
formed at the junction of NR1 and NR2 subunits (hence the need for both subunits
to be expressed). In addition to glutamate, the NMDA receptor requires a
co-agonist, glycine, to bind to allow the receptor to function. The glycine
binding site is found on the NR1 subunit (see Figure 3). The NR2B subunit
also posesses a binding site for ployamines, regulatory molecules that modulate
the functioning of the NMDA receptor. 
At resting membrane potentials, NMDA receptors are inactive. This is due to a voltage-dependent block of the channel pore by magnesium ions, preventing ion flows through it. Sustained activation of AMPA receptors by, for instance, a train of impulses arriving at a pre-synaptic terminal, depolarises the post-synaptic cell, releasing the channel inhibition and thus allowing NMDA receptor activation . Unlike GluR2-containing AMPA receptors, NMDA receptors are permeable to calcium ions as well as being permeable to other ions. Thus NMDA receptor activation leads to a calcium influx into the post-synaptic cells, a signal thought to be crucial for the induction of NMDA-receptor dependent LTP and LTD (animation).
In addition to variation produced by the use of different
NR2 subunits in the receptor complex, the NR1 subunits exists as multiple
splice variants, produced by differential splicing of the mRNA derived from
a single gene. There are two sites for splice variation, in the N-terminus
and the C-terminus (click the indicated regions on Figure 4 for details).
Such splice variation may be important in the regulation of intracellular
interactions such as those with PDZ
binding proteins such as PSD-95. A protein previously known as NMDAR-L
has also recently been shown to be a NMDA receptor subunit, now termed NR3A.
AMPA receptors mediate fast synaptic transmission in the CNS and are composed of subunits GluR1-4, products from separate genes. Like all the ionotropic glutamate receptors subunits, but in contrast to other ligand gated ion channel subunits such as those forming the GABAA receptor, GluR subunits have an extracellular N-terminus and an intracellular C-terminus (illustrated by GluR2, Figure. 5). The ligand binding domain is made up from N-terminal regions S1 and S2 (although, like the NMDA receptor, it is possible that the binding site is spread across more than one subunit), while the C-terminus contains binding sites for proteins such as NSFand PICK1. Work currently underway at the Centre for Synaptic Plasticity by Graham Collingridge, Jeremy Henley, Elek Molnar, and John Issac is investigating the role these proteins play in AMPA receptor trafficking and targetting. For details of interacting partners of other glutamate receptors, click here.
All AMPA receptor subunits exist as two splice variants, flip and flop. The alternative splice cassette is found at the C-terminal end of the loop between TMIII and TMIV. Although the change in the receptor subunits is small (only a few amino acids are changed), the effect can be quite dramatic, resulting in altered desensitisation kinetics.
Native AMPA receptor channels are impermeable to calcium,
a function controlled by the GluR2 subunit. The calcium permeability of the
GluR2 subunit is determined by the post-transcriptional editing of the GluR2
mRNA, which changes a single amino-acid in the TMII region from glutamine
(Q) to arginine (R). This is the so called Q/R editing site - GluR2(Q) is
calcium permeable whilst GluR2(R) is not. Almost all the GluR2 protein expressed
in the CNS is in the GluR2(R) form, giving rise to calcium impermeable AMPA
receptors. This, along with the interactions with other intracellular proteins,
makes GluR2 perhaps the most important AMPA receptor subunit.
Kainate receptors constitute a separate group from the NMDA and AMPA receptors, although they share many of the same structural characteristics. They are built from multimeric assemblies of GluR5-7 and KA-1/2 subunits. Like the other ionotropic glutamate receptors, they possess an extracellular N-terminus that, together with a loop between TMIII and TMIV, forms the ligand binding domain and a re-entrant loop (TMII) that forms the lining of the pore region of the ion channel. They also undergo both splice variation (click on marked regions on Figure 6 for details) and RNA editing, giving rise to a large number of possible receptors with differing pharmacological and functional properties (Figure 6). Further information regarding many of the aspects of kainate receptor structure and function was recently published (Chittajallu et al (1999) TiPS 20; 26-35 y).
Until recently, little was known about the functional and physiological
roles of kainate receptors in the mammalian CNS.For
a review of the current state of knowledge regarding the functional roles
of kainate receptors, click here or on
Figure 6.
Metabotropic
Glutamate Receptors 
Metabotropic glutamate (mGlu) receptors are G-protein coupled receptors (GPCRs) that have been subdivided into three groups, based on sequence similarity, pharmacology and intracellular signalling mechanisms (Figure 7). Group I mGlu receptors are coupled to PLC and intracellular calcium signalling, while group II and group III receptors are negatively coupled to adenylyl cyclase.
Together with the GABAB receptor, the mGlu receptors
form a second family of receptors that are distinct from the adrenergic-type
GPCRs. While both receptor superfamilies possess a 7-transmembrane domain
motif, the mGlu receptors are much larger and agonist binds in the large
N-terminal domain (Figure 8). Again, alternative splice variants are also
found for each mGlu receptor.
Much
effort in recent years has gone into the development of group selective
and subtype selective pharmacological agents to use as tools for the study
of these receptors. This has lead to the synthesis of a range of compounds,
such as 3,5-DHPG and CHPG (group I and mGlu5 selective agonists, respectively)
and LY354740 (group II selective agonist) with which to study the
functions of these receptors in the CNS (for a comprehensive review, see
Schoepp, Jane & Monn (1999) Neuropharmacology 38; 1431-1436).
Members of the MRC Centre have been heavily involved in this process.
The trafficking, targetting and localisation of the mGlu receptors is of great interest. Work from the MRC Centre has recently shown that a GFP fusion protein of the long isoform of mGlu1 (mGlu1a-GFP) has recently been shown to be internalised in response to agonist.
The location of mGlu1a in the post-synaptic membrane is also highly specific. It is found surrounding, but not within the post-synaptic density. This very precise localisation is likely to be brought about by regulatory proteins which bind to the intracellular portions of the receptors. A likely candidate for these proteins are the Homer family of small, PDZ domain-containing proteins, that have recently been shown to interact with group I mGlu receptors.
Metabotropic glutamate receptors play multiple
roles in synaptic plasticity. mGlu receptors are involved in the molecular
switch and LTP induction, potentiation of NMDA receptor activity in
CA1 neurones and in hippocampal NMDA receptor-dependent LTD.
GABA is the major inhibitory neurotransmitter in the mammalian CNS and, like glutamate and other transmitters, acts via both ligand gated ion channels (GABAA receptors) and G-protein coupled (GABAB) receptors. GABAA receptors are members of the ionotropic receptor superfamily which includes a-adrenergic and glycine receptors, while the GABAB receptor is a member of the receptor superfamily including the mGlu receptors.
The structure of the GABAB receptor is related to that of the mGlu receptors, with a long extracellular N-terminus containing the ligand binding domain, 7-transmembrane domain motif and intracellular C-terminus. Initially cloned in 1997 (Kaupmann et al. (1997) Nature 386; 239-246) as a two splice variants from a single gene product (now termed GABABR1a/b), it has since been shown to require dimerisation with a second subtype, GABABR2, for functional expression.
PICK1 is a PDZ domain-containing protein, first described as a Protein Interacting
with C Kinase. It has recently been shown to interact via
it's PDZ domain with the extreme C-terminus of the AMPA receptor subunit
GluR2, resulting in the clustering of this subunit into intracellular membrane
compartments (Dev et al, (1999) Neuropharmacology 38; 635-644).
Last updated: January 4, 2007
