Thermodynamics and structural analysis of positive allosteric modulation of the ionotropic glutamate receptor GluA2

Positive allosteric modulators of the ionotropic glutamate receptor-2 (GluA2) are promising compounds for the treatment of cognitive disorders, e.g. Alzheimer's disease. These modulators bind within the dimer interface of the ligand-binding domain and stabilize the agonist-bound conformation slowing receptor desensitization and/or deactivation. Here, we employ isothermal titration calorimetry to determine binding affinities and thermodynamic details of binding of modulators of GluA2. A mutant of the ligand-binding domain of GluA2 (LBD-L483Y-N754S) that forms a stable dimer in solution was utilized. The potent GluA2 modulator BPAM-97 was used as reference compound. Evidence that BPAM-97 binds in the same pocket as the well-known GluA2 modulator cyclothiazide was obtained from X-ray structures. The LBD-L483Y-N754S:BPAM-97 complex has Kd of 5.6 µM (¿H = -4.9 kcal/mol, -T¿S = -2.3 kcal/mol). BPAM-97 was used in a displacement assay to determine Kd of 0.46 mM (¿H = -1.2 kcal/mol, -T¿S = -3.3 kcal/mol) of the LBD-L483Y-N754S:IDRA-21 complex. The major structural factors increasing the potency of BPAM-97 over IDRA-21 are the increased vdW contacts to primarily Met-496 in GluA2 imposed by the ethyl substituent of BPAM-97. These results add important information on binding affinities and thermodynamic details, and provide a new tool in development of drugs against cognitive disorders.