High-level expression of functional glutamate receptor channels in insect cells

We have expressed glutamate-gated ion channels in Spodoptera frugiperda Sf21 insect cells using a recombinant baculovirus system. Cells infected with recombinant baculoviruses encoding the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)-selective glutamate receptor channel subunits GluR-B and GluR-D displayed specific high-affinity [3H]AMPA binding (apparent dissociation constant Kd of 15 nM for GluR-B and 40 nM for GluR-D) with pharmacological profiles typical of AMPA receptors. The binding reached maximal levels (Bmax of 15-30 pmol per mg of membrane protein) by 3-4 days postinfection. AMPA, glutamate and kainate triggered inward currents in GluR expressing cells, indicating assembly of functional homomeric channels. Formation of heteromeric GluR-B/D channels in doubly-infected cells was evident from the diagnostic current-voltage relations of AMPA-activated whole-cell currents. For the solubilization of the receptor, nonionic detergents Triton X-100, n-octyl-D-glucoside and n-dodecylmaltoside proved most effective. Detergent-solubilized receptor preparations were stable, retained their characteristic ligand-binding properties and bound to immobilized wheat germ lectin, demonstrating the glycosylation of insect cell-expressed GluR subunits. The expression level of 300-400 micrograms of receptor protein per liter of suspension culture should facilitate production of glutamate receptors for biochemical and structural studies.