ABNORMAL N-GLYCOSYLATION OF PROTEINS OF THE GLUTAMATE SYNAPSE IN SCHIZOPHRENIA

Abstract

Recent data suggest abnormalities of glutamate transmission in schizophrenia. While many findings of abnormal glutamate receptor expression in schizophrenia have been published, findings are subtle or even contradictory. These conflicting studies on the expression of these receptors leads to a reconsideration of the “glutamate hypothesis of schizophrenia” as not “too many” or “too few” receptors, but rather one of alterations in the cell biological processes that manage the total pool of receptors. Recent data point to abnormalities of glutamate receptor trafficking, delivery, dendritic localization, recycling, and degradation in the brain in schizophrenia. In this study, we extend our findings of abnormalities of intracellular trafficking of the AMPA subtype of glutamate receptor in schizophrenia. AMPA receptor trafficking starts in the endoplasmic reticulum (ER), in which AMPA receptor subunits are postranslationaly N-glycosylated. N-linked high mannose containing sugars added to AMPA subunits, which are trimmed and replaced by more elaborate sugars in the Golgi, after which AMPA subunits are trafficked for insertion into the plasma membrane. We assayed N-glycosylation status of AMPA subunits in prefrontal cortex in schizophrenia. N-glycosylation was assessed following digestion with endoglycosidase H (Endo H), which removes immature high mannose containing sugars, or with peptide-Nglycosidase F (PNGase F), which removes all N-linked sugars. We found that both GluR2 and GluR4 were sensitive to Endo H and PNGase F treatment, indicating that they are N-glycosylated; neither GluR1 nor GluR3 are N-glycosylated in human brain. GluR2 was found to have less N-linked high mannose and/or hybrid sugars in schizophrenia. This was confirmed by immunoprecipitation of GluR2 and probing with Concavalin A, a mannose-specific lectin. GluR2 immunoprecipitated from schizophrenia cortex was significantly less reactive to Con A comparing to the comparison group. These results indicate that GluR2 is abnormally glycosylated in schizophrenia, consistent with abnormal assembly or trafficking of the AMPA receptor. Abnormalities of glycosylation in schizophrenia may be more extensive, as we also have preliminary results suggesting abnormalities of Nglycosylation of NMDA subunits and two of the glutamate transporters. These results suggest that there are changes in glutamate receptors in schizophrenia that involve abnormalities of intracellular processes that effectively reduce receptor function even though total cellular levels of these receptors may be normal. Such findings are important because they point to the complexity of molecular and intracellular abnormalities in schizophrenia, and highlight novel sites that may be profitably targeted for drug.