Differential effects of lectins on recombinant glutamate receptors

The effects of the lectins concanavalin A, succinyl concanavalin A, wheat-germ agglutinin and soybean agglutinin were studied at recombinant ionotropic glutamate receptors expressed in Xenopus oocytes. Homomeric and heteromeric receptors from each of the three major classes of ionotropic glutamate receptors ($\text{N-}\text{methyl}\text{-}\text{D}\text{-}\text{aspartate}$ (NMDA), α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainate) were studied. The lectins potentiated homomeric configurations of kainate, AMPA and NMDA receptors to a greater degree than the corresponding heteromeric configurations although the rank order of the lectin potentiating effects was the same for both homomeric and heteromeric receptors within a given glutamate receptor class. The most profound effects of the lectins were observed with the kainate receptors; the rank order of potentiating effects of the lectins at the homo- and heteromeric kainate receptors (Glu₆ and Glu₆/KA-2) was concanavalin A > succinyl concanavalin A > wheat-germ agglutinin > soybean agglutinin. At the recombinant Glu₃ and Glu$\text{2}\text{3}$ AMPA receptor complexes, wheat-germ agglutinin and concanavalin A produced the largest enhancements of the glutamate-activated currents followed by succinyl concanavalin A; soybean agglutinin had no significant potentiating effect. Agonist-evoked currents recorded from oocytes expressing the homo- and heteromeric NMDA receptors were only slightly enhanced by concanavalin A and succinyl concanavalin A but not by wheat-germ agglutinin or soybean agglutinin. These results demonstrate that kainate, AMPA and NMDA receptors display dramatic differences in their responses to lectins, and suggest that the receptor-bound oligosaccharide side chains may play different roles in the functional responses mediated by the three major classes of ionotropic glutamate receptors.