Glutamate is the principal excitatory neurotransmitter in the brain, acting through ionotropic and metabotropic glutamate receptors. AMPA-type receptors (AMPARs) are fast ionotropic cation channels located at postsynaptic membranes of excitatory synapses. They assemble as tetramers of GluA1-GluA4 subunits and form homomeric or heteromeric complexes. The hippocampus, essential for learning and memory, is highly sensitive to excitatory imbalance. Circuit hyperactivity drives temporal lobe epilepsy and seizures, whereas reduced AMPAR function contributes to cognitive decline and dementia. AMPARs are thus promising drug targets. Currently, perampanel is the only FDA-approved AMPAR drug, acting as a negative allosteric modulator (NAM) to suppress seizures, but its nonselective action across the brain leads to side effects. To overcome selectivity issues, JNJ-55511118 (JNJ-118), JNJ-61432059 (JNJ-059), and CERC-611 were developed to selectively modulate AMPARs associated with TARPγ8, enriched in the hippocampus. These molecules act as NAMs, reducing AMPAR/γ8 responses. We solved the cryo-EM structure of GluA1/2/γ8 with three compounds and combined it with electrophysiological recordings in HEK293T cells expressing AMPAR/γ8 complexes. We confirmed strong NAM activity on GluA1/γ8 receptors, with reductions in peak current, steady-state, and desensitization. JNJ-059 also inhibited GluA1/2/γ8 heteromers, a physiologically relevant assembly. Unexpectedly, on GluA2/γ8 receptors JNJ-059 displayed a component of positive allosteric modulation (PAM), manifesting as slower desensitization. To dissect this mechanism, we performed mutagenesis on both TARPγ8 and GluA2 and tested JNJ-059 on the generated constructs. Our results show that the PAM effect requires the long β1 loop of TARPγ8, as shortening or replacing it with the TARPγ2 loop abolished slow desensitization, indicating that modulation critically depends on this loop. The dual NAM/PAM actions of JNJ-059 highlight the complexity of receptor-auxiliary subunit interactions and offer structural insights for designing selective AMPAR modulators with therapeutic potential in epilepsy and related disorders.
Ivica et al. (Sun,) studied this question.