Investigation by ion channel domain transplantation of rat glutamate receptor subunits, orphan receptors and a putative NMDA receptor subunit

Among the 18 ionotropic glutamate receptor subunits identified in the mamma lian central nervous system, five (delta1, delta2, GluR7, chi2 and NR3A, fo rmerly called NMDAR-L or chi1) reportedly fail to form functional ion chann els in heterologous expression systems. Four of these subunits, delta1, del ta2, chi2 and NR3A, have not even been shown to bind glutamatergic ligands, relegating them to the status of 'orphan' receptors. We used a domain tran splantation approach to investigate potential functional properties of the putative ion channel domains of four of these subunits. By exchanging ion p ore domains between functional glutamate receptors (GluR1, GluR6 and NMDAR1 ) with known pore properties we first tested the feasibility of the domain swapping method. We demonstrate that ion channel domains can be transplante d between all three functional subfamilies of ionotropic glutamate receptor s. Furthermore, exchange of ion pore domains allows identification of those channel properties determined exclusively by the ion pore. We then show th at transplanting the pore domain of GluR7 into either GluR1 or GluR6 genera tes perfectly functional ligand-gated ion channels that allow characterizat ion of electrophysiological and pharmacological properties of the GluR7 por e domain. In contrast, delta1, delta2 and NR3A do not produce functional re ceptors when their pore domains are transplanted into either the AMPA recep tor, GluR1, the kainate receptor, GluR6, or the NMDA receptor, NMDAR1. We s peculate that the orphan receptors delta1 and delta2, and the NMDA receptor -like subunit NR3A may serve some modulatory function, rather than contribu ting to the formation of ion channels.