Postsynaptic glutamate receptor delta family contributes to presynaptic terminal differentiation and establishment of synaptic transmission.

Synaptic adhesion molecules such as neuroligin are involved in synapse formation, whereas ionotropic transmitter receptors mediate fast synaptic transmission. In mutant mice deficient in the glutamate receptor delta2 subunit (delta2), the number of synapses between granule neurons (GNs) and a Purkinje neuron (PN) in the cerebellum is reduced. Here, we have examined the role of delta2 in synapse formation using culture preparations. First, we found that the size and number of GN presynaptic terminals on a PN in the primary culture prepared from knockout mice were smaller than those in control culture. Next we expressed delta2 in nonneuronal human embryonic kidney (HEK) cells and cocultured them with GNs. Punctate structures expressing marker proteins for glutamatergic presynaptic terminals were accumulated around the HEK cells. Furthermore, HEK cells expressing both delta2 and GluR1, a glutamate receptor subunit forming a functional glutamate-gated ion channel, showed postsynaptic current. Deletion of the extracellular leucine/isoleucine/valine binding protein (LIVBP) domain of delta2 abolished the induction ability, and the LIVBP domain directly fused to a transmembrane sequence was sufficient to induce presynaptic differentiation. Furthermore, a mutant GluR1 whose LIVBP domain was replaced with the delta2 LIVBP domain was sufficient by itself to establish synaptic transmission. Another member of delta glutamate receptor family delta1 also induced presynaptic differentiation. Thus, the delta glutamate receptor subfamily can induce the differentiation of glutamatergic presynaptic terminals and contribute to the establishment of synaptic transmission.