Molecular determinants responsible for differences in desensitization kinetics of AMPA receptor splice variants.

Flip (i) and flop (o) alternatively spliced variants of the four glutamate AMPA receptor subunits (GluR1-4) are differentially expressed in the CNS and can display distinct rates of desensitization that contribute to the heterogeneity of native AMPA receptor-dependent synaptic responses. In the present study, we initially compared the kinetics of desensitization in response to fast application of glutamate (1 mm) for the eight different homomeric recombinant human AMPA receptors (hGluR1-4i and o) heterologously expressed in mammalian cells. Consistent with previous reports on recombinant rat AMPA receptors, the time constants of desensitization between human GluR1i and GluR1o receptors were the same, whereas the flip isoforms for GluR2-4 receptors exhibited significantly slower rates of desensitization compared with the flop isoforms. To identify the molecular determinants responsible for these functional differences, the effects of exchanging amino acid residues in the flip-flop cassette of GluR2i and GluR2o were investigated. Three amino acid residues in the flip-flop region (Thr765, Pro766, and Ser775 in flip and Asn765, Ala766, and Asn775 in flop) were identified that contribute to splice-variant differences in the rate of desensitization. Recent structural data show that these three residues are located on helix J, which forms part of the intradimer interface of AMPA receptor ligand-binding cores, and that the stability of this interface may regulate desensitization. The present results suggest that these three residues may confer differences in flip and flop receptor desensitization rates by directly and/or indirectly influencing the stability of the interface between adjacent subunits.