Transgenic rescue for characterizing orphan receptors: A review of δ2 glutamate receptor

Even though the genomes of several major species have been sequenced, many orphan receptors with unknown ligands and mechanisms of action remain in the CNS. The δ2 glutamate receptor (GluRδ2) is one of such receptors expressed predominantly in the cerebellar Purkinje cells. On the basis of amino acid similarity, it belongs to ionotropic glutamate receptor (iGluR) family, which mediates fast excitatory neurotransmission in the mammalian CNS. Although its null-mutant mice show prominent motor discoordination, the mechanisms by which GluRδ2 participates in the cerebellar functions have been unclear. To gain insight into GluRδ2's mechanisms, we recently generated mice that express either a wild-type or a mutant GluRδ2 transgene, in which the conserved arginine in GluRδ2's N-terminal putative ligand-binding motif was disrupted. By breeding these transgenic mice onto a GluRδ2 ^-/- background, we obtained two transgenic 'rescue' lines. Surprisingly, the mutant GluRδ2 transgene was as effective as the wild-type GluRδ2 in rescuing the GluRδ2-null mice. As the disrupted arginine residue is highly conserved from ancestral bacterial periplasmic amino acid-binding proteins to mammalian iGluRs, we propose that GluRδ2 may not require glutamate-like amino acids and may function in an unconventional manner. This 'transgenic rescue' approach to investigating orphan receptors is a relatively easy but powerful method when a knockout mouse with a distinct phenotype is already available. The advantages and limitations of this approach, together with certain cautions in interpreting the resulting data, are discussed in this review.