DISC 1 regulates N-methyl-D-aspartate receptor dynamics

The neuromodulatory gene DISC1 is disrupted by a t(1;11) translocation that is highly penetrant for schizophrenia and affective disorders, but how this translocation affects DISC1 function is incompletely understood. N-methyl-D-aspartate receptors (NMDAR) play a central role in synaptic plasticity and cognition, and are implicated in the pathophysiology of schizophrenia through genetic and functional studies. We show that the NMDAR subunit GluN2B complexes with DISC1-associated trafficking factor TRAK1, while DISC1 interacts with the GluN1 subunit and regulates dendritic NMDAR motility in cultured mouse neurons. Moreover, in the first mutant mouse that models DISC1 disruption by the translocation, the pool of NMDAR transport vesicles and surface/synaptic NMDAR expression are increased. Since NMDAR cell surface/synaptic expression is tightly regulated to ensure correct function, these changes in the mutant mouse are likely to affect NMDAR signalling and synaptic plasticity. Consistent with these observations, RNASeq analysis of the translocation carrier-derived human neurons indicates abnormalities of excitatory synapses and vesicle dynamics. RNASeq analysis of the human neurons also identifies many differentially expressed genes previously highlighted as putative schizophrenia and/or depression risk factors through large-scale genome-wide association and copy number variant studies, indicating that the translocation triggers common disease pathways that are shared with unrelated psychiatric patients. Altogether, our findings suggest that translocation-induced disease mechanisms are likely to be relevant to mental illness in general, and that such disease mechanisms include altered NMDAR dynamics and excitatory synapse function. This could contribute to the cognitive disorders displayed by translocation carriers. Introduction N-methyl-D-aspartate receptors (NMDAR) are vital for synaptic plasticity and cognitive processes, and are strongly implicated in the pathophysiology of schizophrenia in particular. GRIN2A, encoding the NMDAR GluN2A subunit, © The Author(s) 2018 OpenAccessThis article is licensedunder aCreativeCommonsAttribution 4.0 International License,whichpermits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changesweremade. The images or other third partymaterial in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to thematerial. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Correspondence: J. Kirsty Millar (kirsty.millar@igmm.ed.ac.uk) Centre for Genomic and Experimental Medicine, MRC Institute of Genetics and Molecular Medicine at the University of Edinburgh, Edinburgh, UK Xtuit Pharmaceuticals, Waltham, MA, USA Full list of author information is available at the end of the article. These authors contributed equally: Kyriakos D. Economides, Ellen Grünewald, Paraskevi Makedonopoulou 12 34 56 78 90 () :,; 12 34 56 78 90 () :,; 1 2 3 4 5 6 7 8 9 0 () :,; 12 34 56 78 90 () :,; was recently reported to show genome-wide significant association with schizophrenia, while genomic copy number variants (CNVs) enriched in schizophrenia patients target GRIN1, encoding the GluN1 subunit. Such findings support a role for NMDAR in psychiatric disorders, but direct mechanistic insight is still required. NMDAR function is regulated at many stages, including subunit expression and composition, and dynamic modulation of surface and synaptic levels. The latter can be elicited through control of NMDAR forward trafficking to the plasma membrane, subsequent insertion into synapses, or endocytosis. The obligatory GluN1 subunit is incorporated into all NMDAR, along with other subunit types including GluN2A/B. GluN1 is synthesised in excess and stored within the endoplasmic reticulum (ER), where NMDAR are assembled prior to transportation to the Golgi and onwards to the cell surface. Every stage of NMDAR forward trafficking is tightly modulated to ensure that the required quantity of receptors is present at the cell surface and synapse. Genetic dysregulation of forward trafficking would be predicted to adversely affect the NMDAR function, with knock-on effects for synapse strength and plasticity. DISC1 is disrupted in a large, multi-generation family by a chromosomal t(1;11) translocation that is linked to major mental illness. DISC1 is critical for several processes in the developing and adult brain, and has been connected to NMDAR function through regulation of downstream processes that underlie synaptic plasticity. DISC1 also regulates neuronal microtubulebased cargo transport, including trafficking of mitochondria, synaptic vesicles and messenger RNAs, and associates with the motor protein adaptors TRAK1 and TRAK2. Here we demonstrate direct interaction between GluN1 and DISC1, and association between GluN2B and TRAK1. Applying a novel live-imaging method, we demonstrate that DISC1 regulates dendritic NMDAR motility. Utilising a mouse model of DISC1 disruption by the translocation, we show that mutant mouse neurons exhibit increased NMDAR fast active transport and cell surface/synaptic NMDAR expression. These observations implicate dysregulated NMDAR dynamics/signalling and excitatory synapse dysfunction in the psychiatric disorders displayed by translocation carriers. In further support of a general disease mechanism revealed by the translocation, we find that it impacts the biological pathways highlighted by independent schizophrenia and depression GWAS and CNV studies. Materials and methods Detailed materials and methods for all experiments are provided in the Supplementary information. Essential information is included below. Generation of induced pluripotent stem cells (IPSC) from