Rescue of abnormal phenotypes in δ2 glutamate receptor‐deficient mice by the extracellular N‐terminal and intracellular C‐terminal domains of the δ2 glutamate receptor

The δ2 glutamate receptor (GluRδ2) is expressed predominantly in cerebellar Purkinje cells. GluRδ2 knock-out mice show impaired synaptogenesis and loss of long-term depression (LTD) at parallel fiber/Purkinje cell synapses, and persistent multiple climbing fiber (CF) innervation of Purkinje cells, resulting in severe ataxia. To identify domains critical for GluRδ2 function, we produced various GluRδ2 deletion constructs. Using lentiviral vectors, those constructs were expressed in Purkinje cells of GluRδ2-deficient mice at postnatal day (P) 6 or 7, and rescue of abnormal phenotypes was examined beyond P30. Most constructs failed to rescue the defects of GluRδ2-deficient mice, mainly because they were not efficiently transferred to the postsynaptic sites. However, a construct carrying only the extracellular N-terminal domain (NTD) and the intracellular C-terminal domain (CTD) linked with the fourth transmembrane domain of GluRδ2 (NTD-TM4-CTD) caused incomplete, but significant rescue of ataxia, consistent with relatively better transport of the construct to the synapses. Notably, the expression of NTD-TM4-CTD in GluRδ2-deficient Purkinje cells restored abrogated LTD, and aberrant CF territory in the molecular layer. Although the expression of NTD-TM4-CTD failed to rescue persistent multiple CF innervation of GluRδ2-deficient Purkinje cells, a similar construct in which only TM4 was replaced with a transmembrane domain of CD4 successfully rescued the multiple CF innervation, probably due to more efficient transport of the protein to postsynaptic sites. These results suggest that NTD and CTD are critical domains of GluRδ2, which functions substantially without conventional ligand binding and ion channel structures.