Ion Channel Blockade of NMDA Receptors By Argiotoxins: Identification of Structural Determinats For Subtype Selectivity

The NMDA family of ionotropic glutamate receptors are ligand-gated ion channels that mediate the majority excitatory synaptic transmission in the brain. Dysfunction of NMDA receptor signalling is involved in a range of neurological and psychiatric diseases for which NMDA receptors are considered important drug targets. NMDA receptors are tetrameric assemblies comprised of two glycine-binding GluN1 subunits and two glutamate-binding GluN2 subunits. Four different GluN2 subunits (GluN2A-D) exist with distinct expression and functional profiles. Identification of subunit-selective NMDA receptor agonists, antagonists, or modulators could prove to be both valuable pharmacological tools as well as potential new therapeutic agents.Polyamine toxins are a group of small molecules found in spiders and wasps that are open-channel blockers of NMDA receptors. Polyamine toxins have found valuable use as pharmacological tools based on their high affinity and selectivity, but have not yet been explored as templates for the development of NMDA receptor drugs. Argiotoxin-636 (ArgTX-636) is a polyamine toxin isolated from the venom of the orb weaver spider Argiope lobata. ArgTX-636 is among the most potent inhibitors of NMDA receptors and blocks these in a use- and voltage-dependent manner, presumably by binding to the ion channel region. We have recently shown that modifications within the polyamine tail of ArgTX-636 can control subtype selectivity. In this study, we explore the rational design of ArgTX-636 analogs to produce NMDA receptor inhibitors with marked improvement in subtype selectivity. Systematic variation of the polyamine tail lead to identification of ArgTX analogs with pronounced subtype selectivity, such as 100-fold differential potency between GluN2A, GluN2B, or GluN2C versus GluN2D containing receptors. To study the structural basis for NMDA subtype selectivity, compounds were docked into models of the NMDA receptor ion channels.