Positive and negative allosteric modulation of metabotropic glutamate receptors: emerging therapeutic potential

Abstract

Metabotropic glutamate receptors (mGluRs) modulate neuronal activity in the central and peripheral nervous systems, and since their discovery have attracted considerable attention as putative therapeutic targets for a range of neurological and psychiatric disorders. A number of competitive agonists and antagonists acting at the N-terminal glutamate binding site have been identified, the majority of which are conformationally constrained or substituted amino acid analogues. These ligands have greatly facilitated investigation of the physiological and pathological roles of the receptor family. However, their utility and therapeutic potential has been restricted by relatively poor bioavailability and central nervous system (CNS) penetration, as well as limited chemical tractability and, generally, a lack of selectivity for individual mGluRs. Recently, a number of non-competitive mGluR ligands have been identified which bind within the receptor transmembrane heptahelical domain. These include both positive and negative allosteric modulators. Positive allosteric modulators do not exhibit intrinsic agonism but facilitate agonist-mediated receptor activity. Negative allosteric modulators include both non-competitive antagonists and inverse agonists. Allosteric modulation offers the potential for improved selectivity, particularly for individual receptors within the mGluR family, and enhanced chemical tractability relative to competitive agonists/antagonists. In addition, positive allosteric modulation provides a distinct, and perhaps superior, profile to receptor agonism, offering the potential for facilitation of physiologically appropriate receptor activation with reduced liability for receptor desensitisation and/or tolerance. Thus, the emerging field of positive and negative allosteric modulation of the mGluR family offers considerable promise for the development of novel therapeutics.

Introduction

Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS) and activates both ionotropic receptors, comprising the α-amino-3-hydroxy-5-methyl-4-isoazolepropionic acid (AMPA), N-methyl-d-aspartate (NMDA), as well as kainate receptor families and a family of G-protein-coupled receptors (GPCR), called metabotropic glutamate receptors (mGluRs). Whilst fast excitatory neurotransmission is typically mediated by the ionotropic receptors, the mGluR can modulate neuronal excitability and synaptic efficacy (Anwyl, 1999). To date, eight mGluRs have been cloned (mGluR 1–8) which can be divided into three groups on the basis of their sequence homology, pharmacology, and signal transduction: group I (mGluR 1 and 5); group II (mGluR 2 and 3); and group III (mGluR 4, 6, 7, and 8). A number of splice variants have been reported for group I and III mGluRs (Pin et al., 2003). The group I mGluRs couple via G_q to phospholipase C whereas the group II and group III receptors couple via G_i/G_o to inhibition of adenylyl cylase activity. In addition, mGluR can modulate the activity of ion channels including Ca²⁺ and K⁺ channels either via the direct action of the βγ subunits of the heterotrimeric G-protein or as a result of stimulation of intracellular signalling pathways (Anwyl, 1999).

The mGluR family members exhibit distinct expression patterns in the mammalian CNS and are expressed in both neuronal and glial cells (reviewed by Shigemoto & Mizuno, 2000). With the exception of mGluR6, which is restricted to the retina (Nakajima et al., 1993), all of the mGluRs are expressed in the rat brain and spinal cord (Shigemoto & Mizuno, 2000, Thomas et al., 2001). In addition, mGluRs are expressed in the periphery, for example on nociceptive primary afferent nerve terminals (Bhave et al., 2001) and C fibres (Walker et al., 2001). Group I mGluRs are typically localised postsynaptically in somatodendritic domains of neurons whereas group II and III receptors are typically presynaptic, present in axonal domains and axon terminals where they can regulate neurotransmitter release (Cartmell & Schoepp, 2000).

The mGluRs, which form functional homodimers (Romano et al., 1996), are members of the GPCR family 3, which also includes the receptors for extracellular calcium, pheromones, and γ-aminobutyric acid (GABA_B receptors; Pin et al., 2003). The mGluRs possess a large bi-lobed extracellular N-terminal domain, which has been demonstrated by both site-directed mutagenesis (O'Hara et al., 1993, Malherbe et al., 2001, Rosemund et al., 2002, Sato et al., 2003) and X-ray crystallography (Kunishima et al., 2000, Tsuchiya et al., 2002) to contain the orthosteric agonist binding site. This agonist-binding domain is linked via a cysteine-rich region to the transmembrane heptahelical domain responsible for G-protein activation (Fig. 1; Bhave et al., 2003, Pin et al., 2003). Thus, in marked contrast to the classical family 1 GPCRs such as rhodopsin, which bind their ligands within a pocket in the transmembrane heptahelical domain, the mGluRs are activated following agonist binding within the extracellular N-terminal region. Ligand binding stabilises a closed conformation of the bi-lobed extracellular structure and promotes rotation of one of the extracellular domains of the receptor dimer relative to the other, triggering intracellular signal transduction possibly by stabilising the two transmembrane heptahelical domains in an active conformation (Jingami et al., 2003). The C-terminus of the mGluRs also plays an important role in the regulation of receptor function through interaction with intracellular proteins including the Homer family (Xiao et al., 2000) and calmodulin (O'Connor et al., 1999).