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

Abstract

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 ␦2 subunit (␦2), 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 ␦2 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 ␦2 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 ␦2 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 ␦2 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 ␦2 LIVBP domain was sufficient by itself to establish synaptic transmission. Another member of ␦ glutamate receptor family ␦1 also induced presynaptic differentiation. Thus, the ␦ glutamate receptor subfamily can induce the differentiation of glutamatergic presynaptic terminals and contribute to the establishment of synaptic transmission.cerebellum ͉ granule neuron ͉ Purkinje neuron ͉ synapse formation S ynapse formation requires the accumulation and organization of multiple proteins on both the pre-and postsynaptic sides (1, 2). Previous studies have shown that expression of postsynaptic neuroligin in a nonneuronal cell triggers presynaptic differentiation in a contacting axon through interaction with presynaptic neurexin (3, 4). The involvement of other synaptic adhesion proteins, such as SynCAM and cadherin, in the formation or maintenance of synaptic structures has also been reported (5-7). For synaptic function, the most important postsynaptic proteins are receptors for neurotransmitters. Glutamate is the most prevalent neurotransmitter in the central nervous system, and there are ionotropic glutamate receptors (iGluRs) and metabotropic glutamate receptors on the postsynaptic membrane. iGluR opens its pore domain to allow the permeation of cations when bound by glutamate, and mediates fast excitatory synaptic transmission (8, 9).So far, iGluR has not been shown to be directly related to synapse formation. However, in mutant mice deficient in the glutamate receptor ␦2 subunit (␦2), the number of synapses between granule neurons (GNs) and a Purkinje neuron (PN) is reduced, suggesting the involvement of ␦2 in synapse formation or maintenance (10, 11). ␦2 is selectively expressed in cerebellar PNs and has been classified as a member of the...

The glutamate receptor subunit delta2 is capable of gating its intrinsic ion channel as revealed by ligand binding domain transplantation

“…The slight reduction in molecular weight for S1, S2, and LBD constructs compared to wild-type GluR6 reflects the deletion of glycosylation sites (one each in S1 and S2) upon domain transplantation. support synapse formation (11,12) while the C terminus was implicated in LTD induction (38). As basal synaptic transmission appears to be normal in delta2 knockout mice, no physiological evidence could yet be obtained that delta2 serves as an ion channel (39).…”

“…More precisely, parallel fiber synapses of cerebellar Purkinje cells, the site of specific and abundant expression of delta2, exhibit reduced long-term depression (LTD) and synapse formation with presynaptic granule cells (3)(4)(5)(6). Recent work suggests that delta2 supports LTD induction via metabotropic signaling through its C terminus (7)(8)(9)(10) and that synapse formation is supported via the subunit's N-terminal domain (11,12). As previous domain transplantations indicated defunct delta receptor ion pores (13), the notion formed that the receptors might not be ion channels (9).…”

Glutamate Receptor Ion Channels: Structure, Regulation, and Function

“…Transgenic experiments show that insertion of a mutant GluD2 into GluD2(Ϫ/Ϫ) mice can rescue these mice from neurological deficits even when the inserted GluD2 has a mutation in the ion channel pore that either disrupts Ca 2ϩ permeability (Kakegawa et al, 2007a) or abolishes current flow through the ion channel (Kakegawa et al, 2007b). In addition, GluD2 can induce presynaptic terminal differentiation even without its LBD, which contains the D-serine binding site (Kuroyanagi et al, 2009;Torashima et al, 2009). These data suggest that GluD2 does not influence cerebellar function through actions as a ligand-gated ion channel.…”

Glutamate Receptor δ2 Associates with Metabotropic Glutamate Receptor 1 (mGluR1), Protein Kinase Cγ, and Canonical Transient Receptor Potential 3 and Regulates mGluR1-Mediated Synaptic Transmission in Cerebellar Purkinje Neurons

“…Morphological studies reveal that the loss of GluR␦2 also causes misalignment of parallel fiber presynaptic active zones with Purkinje cell postsynaptic densities (PSDs) (Kashiwabuchi et al, 1995;Kurihara et al, 1997;Takeuchi et al, 2005). This latter observation is explained by a synaptogenic activity of GluR␦2 Kuroyanagi et al, 2009); the extracellular N-terminal domain of GluR␦2 links to neurexin through the secreted synapse organizer cerebellin-1 (Matsuda et al, 2010;Uemura et al, 2010;Joo et al, 2011;Matsuda and Yuzaki, 2011).…”