Modulation of NMDA receptor surface expression by DISC1 and its pathway partners

Disrupted in Schizophrenia 1 (DISC1) is a well supported risk factor for schizophrenia, bipolar disorder and major recurrent depression. DISC1 is a multifunctional multicompartmentalised scaffold protein with essential roles in neuronal proliferation, differentiation, migration and integration. DISC1 also modulates pathways of vital importance for neuronal signalling and plasticity. One of the major hypotheses for the cause of psychiatric illness is N-methyl-D-aspartate (NMDA) receptor hypofunction. It was observed that NMDA receptor antagonists can induce symptoms of schizophrenia in unaffected individuals, and exacerbate symptoms in patients with schizophrenia. Recent work in our laboratory showed that DISC1 complexes with NMDA receptors within the cell body and at synapse of neurons. Here I studied whether DISC1, or DISC1 missense variants, affect the trafficking of NMDA receptors. This was done by quantifying surface NMDA receptor expression in the presence of DISC1 or variant DISC1. I found that one common variant, 607F, causes a significant reduction in surface expressed NMDA receptors. I went on to show that DISC1 reduces the number of internalised receptors associating with early RAB5-containing endosomes. This indicates that DISC1 may be involved in the trafficking and recycling of NMDA receptors, a process that may be affected by the missense DISC1 variant 607F. Further to this I studied the effects on NMDA receptor trafficking of DISC1 pathway partners Nuclear Distribution Element 1 (NDE1) and Trafficking-protein kinesin binding 1 (TRAK1), both regulators of neuronal intracellular trafficking. Phosphorylation of NDE1 at T131 has been shown to be modulated by DISC1. Using phospho-mimic and phospho-dead NDE1 expression constructs I observed a significant reduction in the surface-expressed NMDA receptors in cells expressing the phospho-mimic form of NDE1. NDE1 may therefore be involved in the trafficking of NMDA receptors, and this role may be modulated by phosphorylation of NDE1. Finally, TRAK1 was shown to associate robustly with the GluN2B subunit, and to decrease the surface expression of NMDA receptors, most likely by sequestering them. The TRAK1-induced GluN2B sequestration may be an artefact, but the association of the trafficking molecule TRAK1 with this subunit may point towards a role in NMDA receptor trafficking. These proteins have been shown to associate with each other and may form a complex in order to traffic NMDA receptors. Disruption of this complex by defective DISC1 expression may affect NMDA receptor trafficking. In the brain this could conceivably contribute to NMDA receptor hypofunction and the development of psychiatric illness

DISC1 complexes with TRAK1 and Miro1 to modulate anterograde axonal mitochondrial trafficking

Disrupted-In-Schizophrenia 1 (DISC1) is a candidate risk factor for schizophrenia, bipolar disorder and severe recurrent depression. Here, we demonstrate that DISC1 associates robustly with trafficking-protein-Kinesin-binding-1 which is, in turn, known to interact with the outer mitochondrial membrane proteins Miro1/2, linking mitochondria to the kinesin motor for microtubule-based subcellular trafficking. DISC1 also associates with Miro1 and is thus a component of functional mitochondrial transport complexes. Consistent with these obser-vations, in neuronal axons DISC1 promotes specifically anterogrademitochondrial transport. DISC1 thus parti-cipates directly inmitochondrial trafficking, which is essential for neural development and neurotransmission. Any factor affecting mitochondrial DISC1 function is hence likely to have deleterious consequences for the brain, potentially contributing to increased risk of psychiatric illness. Intriguingly, therefore, a rare putatively causal humanDISC1 sequence variant, 37W, impairs the ability of DISC1 to promote anterogrademitochondrial transport. This is likely related toanumberofmitochondrial abnormalities inducedbyexpressionofDISC1-37W, which redistributes mitochondrial DISC1 and enhances kinesin mitochondrial association, while also altering protein interactions within the mitochondrial transport complex

DISC1 regulates N-methyl-D-aspartate receptor dynamics:abnormalities induced by a Disc1 mutation modelling a translocation linked to major mental illness

Abstract 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