Alteration of NMDA receptor trafficking as a cellular hallmark of psychosis

Schizophrenic spectrum disorders (SCZSD) are severe mental illnesses with a substantial burden. Research suggests a dysfunctional interplay between genetic and environmental factors disrupting brain network functions, originating during childhood. The glutamatergic hypothesis of SCZSD focuses on N-methyl-D-aspartate receptor (NMDAR) dysfunction, supported by altered NMDAR gene expression and protein levels in post-mortem brains, psychotic-like behaviors induced by NMDAR antagonists, and psychotic-like behavioral deficits in rodents with NMDAR ablation. While mutations in NMDAR subunits are rarely found, genome-wide association studies highlight alterations in synaptic signaling proteins. The molecular and cellular mechanisms remain unclear. NMDARs are heterotetramers with varying subunit compositions (GluN1 with GluN2A-D and/or GluN3) affecting biophysical and pharmacological properties. Developmental changes in NMDAR composition influence synapse plasticity and neuronal network formation. NMDARs undergo constant trafficking, influencing synaptic long-term potentiation and memory. Autoantibodies against NMDAR (NMDAR-Ab) in some patients alter NMDAR surface dynamics, but their prevalence and causality in psychosis are debated. Animal models of psychosis, including pharmacological manipulations, genetic manipulations of susceptibility genes (e.g., DISC1), and developmental manipulations, have been used to investigate the pathophysiology of psychotic disorders. This study aims to identify NMDAR trafficking alterations as a convergence point between various psychosis models.

Extensive research supports the involvement of glutamatergic transmission, particularly NMDA receptor (NMDAR) dysfunction, in the etiology of psychotic disorders like schizophrenia. Studies have shown altered NMDAR gene expression and protein levels in post-mortem brains of schizophrenia patients. Furthermore, NMDAR antagonists can induce psychosis-like symptoms in both humans and rodents. Genetic studies have identified several susceptibility genes, including DISC1, whose disruption is associated with impaired NMDAR-mediated synaptic transmission and plasticity. Developmental models, such as prenatal exposure to methylazoxymethanol acetate (MAM), have also shown alterations in synaptic and network functions leading to psychosis-like behavioral deficits in adulthood. Despite this research, the specific molecular and cellular mechanisms underlying NMDAR dysfunction in psychosis remain elusive. This study focuses on NMDAR trafficking as a potential unifying mechanism across different models of psychosis.

The study utilized a multi-faceted approach involving human cerebrospinal fluid (CSF), genetic manipulations in rodents, and a developmental model. CSF was collected from patients with SCZSD, affective spectrum disorders (AffectSD), subarachnoid hemorrhage (SAH), brain polytrauma, and hemophagocytic lymphohistiocytosis (HLH). Rodent studies involved genetic manipulations (DISC1 knockdown and IL1RAPL1 knockout) and a developmental model using prenatal MAM exposure in rats. Hippocampal neuronal cultures were used to study NMDAR trafficking using single nanoparticle tracking (using Quantum Dots), electrophysiology (extracellular field EPSP recordings and whole-cell patch-clamp recordings), immunocytochemistry (surface and intracellular labeling of NMDAR subunits and other synaptic proteins), Western blot analysis (to quantify NMDAR subunit levels), and behavioral assays (prepulse inhibition, PPI). For the developmental model, MAM was injected into pregnant rats on embryonic day 17 (E17), and NMDAR trafficking was examined at different developmental stages. Stereotaxic injections of competing ligands (TAT-NS and TAT-2B peptides) and antibodies against the GluN2B subunit were used to manipulate NMDAR surface dynamics in the developing rat brain, and the effects on adult PPI were assessed. Data analysis included statistical tests such as Kruskal-Wallis, Dunn's multiple comparison test, Kolmogorov-Smirnov test, ANOVA, and Student's t-test.

The study found consistent alterations in NMDAR trafficking across multiple models of psychosis. CSF from SCZSD patients, but not other conditions, increased the surface dynamics of synaptic NMDAR, reducing cluster area. DISC1 knockdown also reduced NMDAR surface dynamics, while IL1RAPL1 knockout did not. In the MAM model, NMDAR surface dynamics were impaired during peak synaptogenesis (postnatal day 12), with increased synaptic NMDAR and reduced extrasynaptic GluN2B receptors. This was associated with increased amplitude and altered kinetics of NMDAR-mediated EPSCs, and impaired NMDAR-dependent long-term potentiation (LTP). Manipulating NMDAR surface dynamics using competing ligands or antibody-induced receptor cross-linking during the second postnatal week either prevented or rescued the PPI deficit in adult MAM rats, indicating a critical role for NMDAR trafficking in the development of psychosis-related behaviors. These data suggest that alterations in NMDAR surface dynamics contribute significantly to the pathophysiology of psychosis.

This study provides strong evidence supporting the hypothesis that altered NMDAR trafficking is a common cellular hallmark of psychosis. The consistent findings across different models (CSF from SCZSD patients, DISC1 genetic manipulation, and MAM developmental model) demonstrate the robustness of this effect. The success of manipulating NMDAR surface dynamics to prevent or rescue behavioral deficits in the MAM model suggests that targeting NMDAR trafficking may be a promising therapeutic strategy for psychosis. The specific molecules in SCZSD CSF causing altered NMDAR dynamics require further investigation, with potential contributors including altered levels of NMDAR agonists, co-agonists, and cytokines. The study highlights the importance of the early postnatal period (second postnatal week in rodents) as a critical window for interventions targeting NMDAR trafficking. Future research could focus on identifying specific molecular mechanisms and developing novel therapeutic agents targeting NMDAR trafficking pathways.

The study demonstrates consistent alterations in NMDA receptor (NMDAR) trafficking across various models of psychosis. These alterations are implicated in the development of psychosis-related behavioral deficits, highlighting the potential of targeting NMDAR trafficking as a therapeutic strategy. Further research is necessary to understand the specific molecular mechanisms involved and to develop novel therapeutic approaches based on these findings.

The study primarily focused on hippocampal neurons, which might not fully represent the complexity of NMDAR trafficking across the brain. The causality between altered NMDAR trafficking and psychosis requires further investigation. While the study used multiple models of psychosis, the generalizability to all forms of psychosis requires more research.