Ionotropic orphan delta receptors (GluD) are not gated by glutamate or any other endogenous ligand but are grouped with ionotropic glutamate receptors based on sequence similarity. GluD1 receptors play critical roles in synaptogenesis, synapse maintenance and have been implicated in neuronal disorders including schizophrenia, cognitive deficits, and cerebral ataxia. Here we report cryo-electron microscopy structures of the rat GluD1 receptor complexed with calcium and the ligand 7-chlorokynurenic acid, elucidating molecular architecture and principles of receptor assembly. The structures reveal a non-swapped architecture at the extracellular amino-terminal (ATD) and ligand-binding domain (LBD) interface. This is in contrast to other families of ionotropic glutamate receptors (iGluRs) where the dimer partners between the ATD and LBD layers are swapped. Our results demonstrate that principles of architecture and symmetry are not conserved between delta receptors and other iGluRs and provide a molecular blueprint for understanding the functions of the "orphan" class of iGluRs.

GluK3-kainate receptors are atypical members of the iGluR family that reside at both the pre- and postsynapse and play a vital role in the regulation of synaptic transmission. For a better understanding of structural changes that underlie receptor functions, GluK3 receptors were trapped in desensitized and resting/closed states and structures analyzed using single particle cryo-electron microscopy. While the desensitized GluK3 has domain organization as seen earlier for another kainate receptor-GluK2, antagonist bound GluK3 trapped a resting state with only two LBD domains in dimeric arrangement necessary for receptor activation. Using structures as a guide, we show that the N-linked glycans at the interface of GluK3 ATD and LBD likely mediate inter-domain interactions and attune receptor-gating properties. The mutational analysis also identified putative N-glycan interacting residues. Our results provide a molecular framework for understanding gating properties unique to GluK3 and exploring the role of N-linked glycosylation in their modulation.

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GluD2 receptors belong to the orphan delta receptor family of glutamate receptor ion channels. These receptors play key roles in synaptogenesis and synaptic plasticity and are associated with multiple neuronal disorders like schizophrenia, autism spectrum disorder, cerebellar ataxia, intellectual disability, paraplegia, retinal dystrophy, etc. Despite the importance of these receptors in CNS, insights into full-length GluD2 receptor structure is missing till-date. Here we report cryo-electron microscopy structure of the rat GluD2 receptor in the presence of calcium ions and the ligand 7-chlorokynurenic acid, elucidating its 3D architecture. The structure reveals a non-swapped architecture at the extracellular aminoterminal (ATD), and ligand-binding domain (LBD) interface similar to that observed in GluD1; however, the organization and arrangement of the ATD and LBD domains are unique. While our results demonstrate that non-swapped architecture is conserved in the delta receptor family, they also highlight the differences that exist between the two member receptors; GluD1 and GluD2.

Glutamate delta-1 (GluD1) and delta-2 (GluD2) receptors belong to the orphan GluD subfamily of ionotropic glutamate receptors (iGluRs). GluDs were classified as ionotropic glutamate receptors based on their sequence similarity. Two decades after these GluDs were first cloned they are still considered "orphan" due to a lack of knowledge of the endogenous ligands that can activate them. Nevertheless, they are crucial for synapse formation, maturation and maintenance of CNS functions, and are implicated in multiple neuronal disorders, including schizophrenia, autism spectrum disorder and depressive disorders. Over the last decade significant discoveries have been made, include role of GluD receptors in mediating trans-synaptic interactions and their unique non-swapped architecture, which is distinct from other ionotropic glutamate receptors. Also, the prospect of GluD ionotropic activity being regulated by direct interaction with metabotropic glutamate receptors is exciting. These discoveries will likely drive the field in the future, providing direction to GluD research.