Mechanism and subunit-dependence of disease-associated lurcher mutation in AMPA-type glutamate receptors

Abstract

AMPA receptors are glutamate-activated neuroreceptors responsible for the majority of fast excitatory neurotransmission in vertebrate brains. Activity-dependent changes in their functional and trafficking properties underpin cognitive processes such as learning and memory formation. In the past 15 years, a number of mutations within AMPA receptor genes have been identified as disease-causing leading to some form of neurodevelopmental delay. Here, we focused on one such mutation called lurcher, resulting from a change of the third Ala in the highly conserved SYTANLAAF motif to Thr. We determined functional consequences of lurcher mutation in homomeric AMPA receptors composed of subunits GluA1 or GluA2. In GluA1 subunit, lurcher mutation has been identified as a hotspot in a large cohort of patients with neurodevelopmental disorders. We confirm previously established lurcher phenotype characterised by slower deactivation and desensitization kinetics as well as more profound block of desensitization compared to the wild-type (WT) homomers. We show that these effects are more pronounced in GluA2 than GluA1 homomers. Lurcher phenotypes are attenuated, but still present, in more physiologically abundant heteromeric AMPA receptors composed of two GluA1 lurcher and two GluA2 WT subunits (GluA1 lc/GluA2 WT). Schild analysis indicates lurcher enhances activity of AMPA receptors by primarily changing the efficiency with which they open their ion channel upon glutamate binding rather than by changing affinity for glutamate. Perampanel, the only widely licensed drug that targets AMPA receptors, is an anti-convulsant that acts as a non-competitive inhibitor. We show it continues to inhibit GluA1 lc/GluA2 WT heteromeric receptors and that its IC50 depends on the auxiliary subunits associated with the heteromers. Specifically, stargazin (TARP {gamma}-2) brings the IC50 for perampanel into the submicromolar range, just like in the WT receptors.