Differences between the GluD1 and GluD2 receptors revealed by GluD1 X‐ray crystallography, binding studies and molecular dynamics

Abstract

Ionotropic glutamate receptors are ligand‐gated ion channels essential for fast excitatory neurotransmission in the brain. In contrast to most other members of the iGluR family, the subfamily of delta receptors, GluD1 and GluD2, does not bind glutamate but glycine/D‐serine. GluD1 is widely expressed in the brain and the inner ear, where it is required for high‐frequency hearing. Furthermore, it has been associated with schizophrenia, autism and depression. X‐ray structures of the ligand‐binding domain (LBD) of GluD2 have been published; however, no high‐resolution structure is available for the ligand‐binding domain of GluD1 (GluD1‐LBD). Here, we report the X‐ray crystal structure of the GluD1‐LBD in its apo form at 2.57 Å resolution. Using isothermal titration calorimetry, we show that D‐serine binds to the GluD1‐LBD in an exothermic manner with a Kd of 160 μm, which is approximately five‐fold greater than at GluD2. Furthermore, we identify Glu822 as a critical determinant of receptor activation in GluD1 A654T. In contrast to studies on the GluD2 lurcher mutant A654T, we did not observe any effect of 1 mm D‐serine on the spontaneous currents at mouse GluD1 A654T by electrophysiological recordings of Xenopus laevis oocytes as previously also reported by others. These results point towards differences in the structure and dynamics between GluD1 and GluD2. Molecular dynamics simulations were employed to address this observation, suggesting that the apo structure of GluD1 is less flexible than the apo structure of GluD2 and that Pro725 in GluD1 may affect the interlobe closure of the ligand‐binding domain of GluD1.

Small‐molecule positive allosteric modulation of homomeric kainate receptors GluK1‐3: development of screening assays and insight into GluK3 structure

“…Instead, we report distances between key lobe D1 and D2 residues (between CA atoms): Glu443 and Asn722 (EN distance) and Thr520 and Ala691 (TA distance) (Fig. 8), as previously reported for delta receptors [49]. The D1‐D2 interlobe contact (EN) has previously been shown to be essential for the efficacy of agonists at iGluRs [50].…”

Loss of activation by GABA in vertebrate delta ionotropic glutamate receptors

“…Four of the above reverse mutations in the LBD caused Rat GluD2 Lc -mutant channels to be closed at rest. This identifies candidate LBD determinants of the differing effects of M3 mutations in different iGluR families ( 11 , 26 , 49 , 50 ). Indeed, despite high conservation of upper-M3 amino acid sequence in various delta iGluRs ( SI Appendix , Fig.…”

“…Like most iGluRs, delta iGluRs are expressed in excitatory synapses, their up- or downregulation leads to developmental disorders or neural malfunction, and when expressed heterologously in mammalian cells or frog oocytes, GluD1 and GluD2 subunits form homotetrameric channels ( 9 ). In stark contrast to other iGluRs, however, under numerous native and heterologous experimental conditions, no current through GluD1 or GluD2 iGluRs is activated by neurotransmitter binding ( 7 , 10 – 12 ). This is despite two major lines of evidence that the ligands D-serine and glycine bind to the canonical iGluR LBD of GluD2 and induce local conformational change.…”

Structural insights into the ligand binding domain of GluD1 and GluD2 receptors

“…In their study [14], Masternak et al succeeded in solving the 3D structure of the LBD of GluD1 (GluD1-LBD). They could establish the organisation of residues known to interact with D-serine.…”

“…In their study [14], Masternak et al . succeeded in solving the 3D structure of the LBD of GluD1 (GluD1‐LBD).…”