Fourth non canonical sites may be targeted
Fourth, non-canonical sites may be targeted. Classical benzodiazepines require the presence of a γ subunit for high-affinity binding, which limits their activity to a specific large pool of receptor isoforms, leaving other isoforms unaffected. In particular, δ subunit-containing receptors, as well as less-studied receptor populations such as θ or ε subunit-containing receptors, are thought to mediate very specific physiological functions and thus could be potentially interesting targets for novel therapeutic approaches . Non-canonical benzodiazepine sites are present on a wide range of receptors because they can also be formed by α and β subunits. Given that the benzodiazepine scaffold exhibits a superbly benign toxicological profile and excellent ADMET (absorption, distribution, metabolism, excretion, and toxicity) properties, developing some degree of specificity for activity at non-canonical sites offers a very valuable avenue to explore novel medicinal chemistry on the background of established drugs.
The GABAA receptor and its drug binding sites may thus be predicted to play a prominent role in the search for treatment of central nervous system diseases.
Introduction N-Methyl-D-aspartic cetrimonium bromide (NMDA) receptors are ligand-gated ion channels important for fast excitatory synaptic transmission, distributed throughout the central nervous system (Moriyoshi et al., 1991, Monyer et al., 1992, Paoletti et al., 2013). They are necessary for learning and memory and are drug targets for treating Alzheimer's disease (McKeage, 2009), depression (Moskal et al., 2014), epilepsy (Hu et al., 2016), and schizophrenia (Balu, 2016). These receptors are obligate heterotetramers, typically comprising two glycine-binding GluN1 subunits and two glutamate-binding GluN2 subunits (Mayer et al., 1984). Each subunit contains an extracellular amino-terminal domain (ATD) and ligand-binding domain (LBD) in addition to a transmembrane domain (TMD) and an intracellular C-terminal domain (CTD) (Mayer, 2017). The LBDs possess an overall clamshell-like conformation in which a hinge separates two lobes of the LBD (lobes 1 and 2) (Figure 1). The TMDs of the four subunits surround a central pore to form the core of the ion channel, and in each subunit, the TMD is connected to lobe 2 of the LBD by three short linkers. Binding of two distinct agonists, glutamate and glycine (or D-serine), to the individual glutamate- and glycine-binding LBDs activates the channel. Agonist binding triggers conformational change in the LBD, causing the two lobes to close around the ligand and is thought to provide the useful work to open the channel pore, which, together with a voltage-dependent unblock of magnesium, allows the entry of calcium into the postsynaptic cell and depolarization of the membrane potential (Mayer et al., 1984, Nowak et al., 1984, MacDermott et al., 1986). Crystallographic and cryo-electron microscopic studies have captured NMDA receptor LBDs bound to glutamate and glycine, as well as various other agonists and antagonists, shedding light on the molecular interactions involved in stabilizing the ligands and conformational changes accompanying ligand binding (Inanobe et al., 2005, Furukawa et al., 2005, Yao et al., 2013, Jespersen et al., 2014, Hackos et al., 2016, Volgraf et al., 2016, Zhu et al., 2016, Tajima et al., 2016, Yi et al., 2016, Villemure et al., 2017, Lü et al., 2017, Romero-Hernandez and Furukawa, 2017, Lind et al., 2017). The precise molecular details of the dynamical processes associated with ligand binding, however, are unknown. The α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are related ionotropic glutamate receptors (iGluRs) that mediate fast excitatory neurotransmission, and are also typically heteromeric ion channels, composed of GluA2 subunits in conjugation with GluA1, GluA3, or GluA4 subunits (Isaac et al., 2007, Herguedas et al., 2016). Like NMDA receptors, they have functionally modular domains separated into the ATDs, LBDs, TMD, and CTDs, but differ in that activation of the channel requires binding of only a single agonist, glutamate, to the separate LBDs. Recent computational and electrophysiology studies have elucidated the binding mechanisms of glutamate to the AMPA receptor and found binding intermediates that assist the process of ligand binding (Yu and Lau, 2017, Yu et al., 2018).