Abnormal N-linked glycosylation of cortical AMPA receptor subunits in schizophrenia

Numerous studies have demonstrated brain region- and subunit-specific abnormalities in the expression of subunits of the AMPA subtype of glutamate receptors in schizophrenia. In addition, abnormalities in the expression of proteins that regulate the forward trafficking of AMPA receptors through the cell have been reported. These findings suggest abnormal trafficking of AMPA receptors as a mechanism underlying dysregulated glutamate neurotransmission in schizophrenia. AMPA receptor subunits (GluR1-4) assemble to form AMPA receptor complexes in the lumen of the endoplasmic reticulum (ER). These subunits undergo the posttranslational modification of N-linked glycosylation in the ER and the Golgi apparatus before the assembled receptors are transported to the plasma membrane. In this study, we measured expression of AMPA receptors and the extent of their N-glycosylation using Western blot analysis in the dorsolateral prefrontal cortex in subjects with schizophrenia (N = 35) and a comparison group (N = 31). N-glycosylation was assessed using molecular mass shift assays following digestion with endoglycosidase H (Endo H), which removes immature high mannose-containing sugars, and with peptide-N-glycosidase F (PNGase F), which removes all N-linked sugars. Of the four AMPA receptor subunits, only GluR4 was significantly increased in schizophrenia. GluR2 and GluR4 were both sensitive to Endo H and PNGase F treatment. Endo H-mediated deglycosylation of GluR2 resulted in a significantly smaller pool of GluR2 protein to shift in schizophrenia, reflecting less N-linked high mannose and/or hybrid sugars on the GluR2 protein in this illness. This was confirmed by immunoisolation of GluR2 and probing with Concanavalin A, a mannose specific lectin; in subjects with schizophrenia GluR2 was significantly less reactive to Concanavalin A. Altered N-linked glycosylation of the GluR2 subunit in schizophrenia suggests abnormal trafficking of AMPA receptors from the ER to the synaptic membrane in schizophrenia.