GluRδ2 Expression in the Mature Cerebellum of Hotfoot Mice Promotes Parallel Fiber Synaptogenesis and Axonal Competition

Glutamate receptor delta 2 (GluR delta 2) is selectively expressed in the cerebellum, exclusively in the spines of the Purkinje cells (PCs) that are in contact with parallel fibers (PFs). Although its structure is similar to ionotropic glutamate receptors, it has no channel function and its ligand is unknown. The GluR delta 2-null mice, such as knockout and hotfoot have profoundly altered cerebellar circuitry, which causes ataxia and impaired motor learning. Notably, GluR delta 2 in PC-PF synapses regulates their maturation and strengthening and induces long term depression (LTD). In addition, GluR delta 2 participates in the highly territorial competition between the two excitatory inputs to the PC; the climbing fiber (CF), which innervates the proximal dendritic compartment, and the PF, which is connected to spiny distal branchlets. Recently, studies have suggested that GluR delta 2 acts as an adhesion molecule in PF synaptogenesis. Here, we provide in vivo and in vitro evidence that supports this hypothesis. Through lentiviral rescue in hotfoot mice, we noted a recovery of PC-PF contacts in the distal dendritic domain. In the proximal domain, we observed the formation of new spines that were innervated by PFs and a reduction in contact with the CF; ie, the pattern of innervation in the PC shifted to favor the PF input. Moreover, ectopic expression of GluR delta 2 in HEK293 cells that were cocultured with granule cells or in cerebellar Golgi cells in the mature brain induced the formation of new PF contacts. Collectively, our observations show that GluR delta 2 is an adhesion molecule that induces the formation of PF contacts independently of its cellular localization and promotes heterosynaptic competition in the PC proximal dendritic domain.